The FIB was authorized by Resolution 2098 (2013); Adopted by the Security Council at its 6943rd meeting, on 28 March 2013 - S.C. Res. 2098, U.N. Doc. S/RES/2013 (28 Mar. 2013). Its most pertinent provisions (besides its basis in Chapter VII "Peace Enforcement", whose first major use was the Korean Police Action of 1950) appear to be in paras 9-12 dealing with the FIB, esp. para 12:

12 ... (a) Protection of civilians

(i) Ensure, within its area of operations, effective protection of civilians under imminent threat of physical violence, including civilians gathered in displaced and refugee camps, humanitarian personnel and human rights defenders, in the context of violence emerging from any of the parties engaged in the conflict, and mitigate the risk to civilians before, during and after any military operation;
...
(b) Neutralizing armed groups through the Intervention Brigade

In support of the authorities of the DRC, on the basis of information collation and analysis, and taking full account of the need to protect civilians and mitigate risk before, during and after any military operation, carry out targeted offensive operations through the Intervention Brigade referred to in paragraph 9 and paragraph 10 above, either unilaterally or jointly with the FARDC, in a robust, highly mobile and versatile manner and in strict compliance with international law, including international humanitarian law and with the human rights due diligence policy on UN-support to non-UN forces (HRDDP), to prevent the expansion of all armed groups, neutralize these groups, and to disarm them in order to contribute to the objective of reducing the threat posed by armed groups on state authority and civilian security in eastern DRC and to make space for stabilization activities; ...
The bolded items delineate some policy issues. First off, how will the UN define "civilians" (which require "effective protection") ? What does "strict compliance" with IHL mean ? What does "neutralize" mean - kill, capture or convert as in the Phoenix Program of Vietnam; or something quite different in the contemplated "targeted offensive operations"?

Within three months of the resolution, a similar set of questions was asked by Bruce 'Ossie' Oswald, The Security Council and the Intervention Brigade: Some Legal Issues (June 6, 2013):

The key provisions concerning the Brigade are found in the operative paragraphs 9, 10 and 12(b) of UNSC resolution 2098. The UNSC mandated the Brigade "to carry out targeted offensive operations . . . with the responsibility of neutralizing armed groups."

The role of the Brigade is also to "prevent the expansion of all armed groups . . . and to disarm them in order to contribute to the objective of reducing the threat posed by armed groups on state authority and civilian security in eastern DRC and to make space for stabilization activities."

Based on the references to the armed groups made elsewhere in the resolution, it is reasonable to assume that the Brigade is mandated to undertake offensive operations against, for example, the M23, the Democratic Liberation Forces for the Liberation of Rwanda, the Lord's Resistance Army, and various Mayi Mayi groups.
...
The UNSC stipulated that the legal framework applicable to the Brigade in carrying out its functions and tasks is international law, including international humanitarian law.
...
Key Legal Issues

The UNSC's innovation in establishing an offensive military force to "neutralize" non-state armed actors in the DRC raises two broad and important legal questions. The first is whether the Brigade, as a matter of law, should legally be considered a party to the conflict in the Congo. The second question concerns the verb "neutralize" and the powers that the Brigade might imply from it.
Oswald is a professor at the Melbourne Law School; and was one of the primary draftsmen of the Copenhagen Process on the Handling of Detainees in International Military Operations (The Copenhagen Process was launched on 11 October 2007 and was concluded in Copenhagen on 19 October 2012).

As to "neutralization", Oswald finds a number of open policy questions:

A further question regarding the establishment of the Brigade is why the UNSC thought it necessary to stipulate that the Brigade is mandated to 'neutralize' the armed groups. Did the UNSC use that word as a term of art to mean that the Brigade should "render [the armed groups] ineffective or unusable"? If yes, did the UNSC intend to distinguish the Brigade's mandate from other operational terms such as "contain," "defeat," "destroy," "disrupt," or "exploit"? If the term is being used in a specific way, what ramifications does that have for the Brigade's functions and tasks? Might "neutralize" mean that targeting or capture of the rebel forces is limited to making the rebels ineffective? Furthermore, it is not clear why the UNSC felt it had to add "neutralize" to the Brigade's mandate when the Brigade, as a subordinate component of MONUSCO, would have the ability to "take all necessary measures" to complete its mandate. Is the Brigade to interpret "take all necessary measures" more narrowly because of the word "neutralize"?

If the term "neutralize" is read broadly, it is reasonable to assume that the Brigade is mandated to target armed groups with lethal force. In line with the usual concept of offensive operations, the Brigade would be able to conduct ambushes, deliberate attacks and hold ground against any armed group. It therefore follows that the Brigade's rules of engagement would be amended to take into account the offensive nature of the operations, and that the international humanitarian law principles of necessity, proportionality, humanity and distinction would be considered accordingly - that is, from a different perspective to how those terms would be applied when self-defense is the justification for the use of force. The rules of engagement presumably would also have to deal with whether the basis for targeting is that a rebel is a member of an organized armed group, or whether he or she is taking an active part in hostilities.
Thus, Oswald and I seem to take a similar broad definition of "neutralize"; however, he would (based on the Copenhagen Process) probably take a more restrictive definition of "combatant", and a more expansive definition of "civilian" than I.

- to be cont. -