Well, it's pretty simple really.
Credit the original authors, whose names are literally on every item's name if inspected in OpenBRF (i.e., if it says it's barf_blahblah in the BRF, guess what, Barf made it originally, and I've modified it in turn) and you can use any of it in a mod. For the most part, this is about as easy as it can be; the artists whose collective work is in the mod is listed very clearly in the Wall of Text amongst other places, so matching up rrr_something with RR_Razor isn't a big deal.
If all of that sounds like "too much work" for you, be warned; I get pretty touchy about this if it comes to light that somebody's been "borrowing" our work without bothering to properly credit us, because all of these people have helped me, either directly or indirectly, and I take my responsibilities as the project's editor / artist-in-chief really seriously.
The safest route to crediting properly, if you're going to use our stuff in a project, is simply to copy-paste the credits from the Wall of Text, entire, into your own public credits. That ensures you don't miss anybody and it's easy. Some of those people on the list were coders and other helpers, but I don't list them separately, because I want everybody to be honored equally- help is help.
Please note that some items, like Dejawolf's, are restricted to use on this engine only; it's up to modders to do their homework.
Lastly, note that a lot of the items have been specifically engineered to make best use of my custom shaders, and won't work the same in Native. I apologize for any inconvenience that may cause, but it was necessary to achieve the look-and-feel that makes the project special.
If you have specific questions, you can ask me about them here, but basically the intent is to let people use this stuff as they please, on this engine.
There are no such restrictions on the source code; all I ask is that people not borrow the name, i.e., no "Blood and Steel Rebal Mod" or "Blood and Steel Realism Mod" stuff, please.