Our Gemara on Amud Aleph describes a certain rabbinic derasha about Metzora as “Miderabanan”. This is an odd text because usually the term Midreabanan refers to laws that are rabbinic additions, not laws derived from derashos. The obvious difference being that something derived from a derasha has the force of a Torah law, and not subject to certain leniencies, such as physical duress, or following the lenient opinion when in doubt.
One of the most debated idea about derashos identified as Miderababan is the famous statement by the Rambam in Hilchos Ishus (1:2) that describes marriage as being effectuated, either through sexual intercourse or through a written contract, or through the exchange of an object of value. The first two, sexual intercourse, and a contract, are described by the Rambam as biblical in origin, while the latter item, the object of value, he describes as coming from the Soferim, a synonymous term for Rabbinic authorities.
This Rambam caused much ink to be spilled, because he seems at first glance to be saying that marriage effectuated through the exchange of an object of value is only rabbinic in origin. The problem with this is that the Gemara Kiddushin (2a-2b) treats marriage effectuated through an exchange of an object of value in an equal fashion to marriage achieved through the other two processes, the difference being that the object of value is derived through a Gezeirah Shava derasha as opposed to more direct scriptural inferences. (A Gezeirah Shavah derasha can be best understood as a kind of hyperlink. Where there are two similar words known by tradition to create a cross reference from one section of the Torah to another. According to the Gemara there was an oral tradition regarding which words have this hyperlink capacity, and the rabbis then used these links to infer different meanings and laws as they correspond and relate, back-and-forth between two sections of the Torah. See Niddah 19b.)
It is fairly clear that this is what the Rambam meant when he described Kesef Kiddushin as coming from the Soferim, and he did not mean in any way to suggest it was of lesser status or less binding. However, that still leaves us with the question of why make the distinction at all? What is the difference between something that is derived as a result of rabbinic explication from the verses according to their principles of derash such as a gezeira shava versus an outright scriptural derivation? We will come back to this later in our discussion.
The Ramban in his commentary on Sefer Hamitzvos (Shoresh Two) challenges the Rambam’s description by stating we do not find anywhere in Rabbinic literature that something derived from a derasha is called divrei soferim.
Maharitz Chayos notes our Gemara here, and quotes the Responsa of the Besamim Rosh (17) who asks that this seems to refute the Ramban, because here indeed our Gemara seems to refer to the derasha by metzora to be Mederabanan. Maharitz Chayos, in his typical blend of modern scholarship and encyclopedic knowledge of Rabbinic sources, notes that the Yerushalmi (Kiddushin chapter 1) has a variant text. In the Yerushalmi, it refers to this as miderasha and not miderabanan. Maharitz Chayos quotes his rebbe (Rabbi Margoliyos), that our Gemara is simply a printer's mistake. The first three letters mem-dalet-reish are the same for both words medrabanan and miderasha. What happened was an original manuscript wrote the first three letters and then had the strechel to note it was an abbreviation (“מדר׳”). The printers erroneously completed the abbreviation as miderabanan and not mederasha. He furthermore linguistically proves this, because the grammar is incorrect. It should have stated “derabanan” from the particular context of the sentence and not “miderabanan”. Therefore, the entire question on the Ramban does not even begin, because there is no such Gemara text which contradicts him.
As a historical note, most historians consider the Sefer Besamim Rosh as an elaborate forgery perpetrated by the Maskil Saul Berlin. Though dozens of rabbinical authorities quote this sefer throughout the generations (and now we can count the Maharitz Chayos as one of them), if you get a hold of an uncensored version you can see that some of these Responsa are clearly Trojan horses, designed to implant heretical and Maskilic ideas under the guise of pedigreed rabbinic responsa. (See Psychology of the Daf Yoma 38, where we find Ben Yehoyada quoting one of these responsa to permit suicide under extreme circumstances.) Our particular responsum could easily be in that category, as it is supporting a view of the Rambam against the Ramban, which can be used for radical purposes, as we shall soon see, but also can be relevant based on how the Rambam is authentically interpreted. Some authorities say that the Besamim Rosh are authentic responsa, with forgeries woven some of them.
Be that as it may, let us return to the Rambam’s original idea, that certain halachos derived from a gezeirah shava are considered Midivrei Soferim, rabbinic, even if they have the full force of Torah law. As we asked earlier, if so, what is the purpose of the distinction? The answer lies in the mechanism and power of the gezeirah shava. As we saw earlier, the Gemara Niddah 19b states:
אין אדם דן גזרה שוה מעצמו
A person may not derive a verbal analogy on his own, but only if he received it by tradition
The question is what are the limits of this? That is, is there a tradition dictating fully what are the laws to be derived and these word connections merely serve as indices and reminders? Or, are the words hyperlinks, but how and which of these are derived, and what particular laws are learned and not are based on some type of Rabbinic Reasoning. Furthermore, if they are based on rabbinic reasoning, is the reasoning only about the law itself, or other considerations such as social needs, overall implications on observance, also taken into account? In other words, shall we view the derivations from these linked words as a merely another form of Oral Torah that is not subject to any particular change or logic, or can each subsequent Sanhedrin apply various considerations in their judicial process, allowing them to create “new” halachos?
This is an old debate, but I would like to add some of my own original observations based on Gemaras and Midrashim, which I will quote:
The Gemara (Yevamos 76b-77a) records a dispute that took place regarding King David’s lineage and eligibility for the throne before his inauguration. Since he descended from Ruth the Moabite, and Moabites are permanently banned from entering the congregation. “An Ammonite or a Moabite shall not enter into the congregation of the Lord” (Devarim 23:4)
Amasa brought a tradition he received from the court of Samuel from Rama: An Ammonite man is prohibited from entering into the congregation, but not an Ammonite woman; a Moabite man is prohibited from entering into the congregation, but not a Moabite woman.
In the Talmudic version of this story, The resolution comes from the testimony of Amasa, who had a tradition from the court of Samuel the prophet. Thus, David’s lineage was safe, as he descended from a Moabite woman, who is permitted to intermarry.
Compare this to the narrative in Pesikta Derav Kahana (16), as well as in Ruth Rabbah (7:7)
Pesikta DeRav Kahana:
Boaz said to Ruth, “You traveled to a nation you had not known yesterday or the day prior”. He meant to say, “If you had come a day or two earlier to Convert, we would not have accepted you (for marriage). As the halakha had not yet been initiated, to declare a male Amonite is forbidden, not a female. And a male Moabite is forbidden, not a female.
Ruth Rabbah:
Meanwhile, Boaz had gone to the gate and sat down there. And now the redeemer whom Boaz had mentioned passed by...Rabbi Joshua said: his name was Ploni Almoni ("so and so")". Rabbi Samuel the son of Nachman said that he was ignorant of the words of the Torah. He said: "the first ones (Ruth’s and Orpah’s husbands) did not die but rather because they took her and I am going to go and take her? I certainly am not going to take her. I will not pollute my seed and I am not going to create unfitness for my children". And he did not know that the halakhah had been initiated: Ammonite and not Ammonitess; Moabite and not Moabitess".
It is evident in contradistinction to the Talmud’s narrative, Ruth Rabbah and Pesikta Derav Kahana understood that the rabbinical court ruled, right at that time, to permit a female Moabite, in order to save David’s lineage. Malbim Ruth (1:22) is explicitly in favor of the narrative from Ruth Rabbah and Pesikta Derav Kahana:
The verse that refers to Ruth as “The one who returns from the fields of Moab.”, means to say she is the first returnee from the fields of Moab. That is, she is the first convert from Moab accepted for intermarriage because up until this point in time, they did not yet derive from the verse, a male Moabite, not a female.
Thus, the Rambam wants us to know the difference in the source of the halakha, because since the aspect of kiddushin via monetary objects is from a gezeirah shava it can be subject to change by a later Sanhedrin. (For more on this, see Kinas Soferim on Sefer HaMitzvos Shoresh Beis discusses this at great length. Also see Introduction of Sefer Dor Revi’i, and for a superb translation by Rav Yaakov Elman Z”L, see here http://wwwarchive.math.psu.edu/glasner/Dor4/elman.html (TRADITION: 25(3), Spring 1991, pp. 63-69.)
The idea that the rabbis of the Sanhedrin, past and future, had flexibility and discretion in regard to gezeirah shava has far reaching implications. Rav Kook in Moreh Le-Nevukhai Hador (13) argues in favor of the idea that in some future time, when a Sanhedrin is established, it will necessarily take upon itself to revise laws as needed to accommodate the social, psychological, moral and developmental needs of that particular time in history. Rav Kook argues that if the Sanhedrin is able to find verses to support a new interpretation, they can do so, and shall do so in response to an important preservatory goal. (This is similar to the Talmudic example of Rabbi Akiva and the niddah laws, see Shabbos 64b and Ben Yehoyada, also see Psychology of the Daf Nedarim 74.)
Other authorities are not so accepting of this position, and it would seem that this was the basis of the Ramban’s objection to the Ramban, as well as the Gemara’s versions of the midrash by David and Ruth. This also gives more insight into why Saul Berlin found it necessary to highlight this Gemara and attempt to refute the Ramban, because in the wrong hands, these insights could lead to a rejection of the validity of Torah law and rabbinic derash, which of course was the farthest thing from the Rambam, Rav Glasner and Rav Kook. A theoretical halakhic position and idea about halacha that can be misused is merely dangerous because it can be misused by heretics, but is not itself heresy.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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