"RC" is short for "Research Chemical," and it is a term that is used to categorize a psychoactive substance that has psychotropic effects and structure comparable to a more traditional drug, but which is novel either because its synthesis was only recently discovered or because it is less well known for some other reason.
With that said, Nbome isn't "on" any research chemical, it is a kind of research chemical. The reason Nbome's are undesirable is several-fold. First, they've been shown to have dangerous and unpredictable physical effects even at dosages that are well within the recreational range. For example, 500mcg of Nbome is a common dose, and you may be completely fine taking it one time and then have a seizure or other serious physical complication the next time. Or your friend may take 500mcg and be fine, but you take it and end up with severe complications.
This brings me to my next point: very few of the currently known psychedelic substances on the market have substantial psychedelic effects at the 500mcg range, but Nbomes do. Such a low dose means that an active dose can fit on blotter paper. LSD is similar to Nbome in this regard and this is what makes things that much more dangerous. For, LSD has been known to be quite physically safe even at doses well north of the recreational dosages. So a kid who has some tabs of LSD can eat a 10 strip and will be physically fine, come what may psychologically. But if you ate a 10-strip of Nbomes, thinking it was LSD because it came on blotter paper, you'd be fucked.
So, in sum, Nbome's are a kind of research chemical that are active at dosages low enough so that the dose can fit on blotter paper. Furthermore, Nbome's are cheap and readily available online. So people can make a massive amount of money redistributing Nbome's under the guise of them being LSD. If your boy sells you tabs of something that isn't LSD, don't accept them because they will most likely be Nbomes (there are other potential options, but not many). Similarly, and this is doubtful, if he tries to sell you a powder for which the active dosage is micrograms instead of milligrams, then it'll be a good chance you're dealing with an Nbome.
On the other hand, if he sells you 2c-b, 2c-i, 2c-e, 2c-d, or anything like that, what you have is not an Nbome. Of course these too need to be treated with the appropriate care, but they seem tobe much, much, much safer than Nbomes as they have not been shown to lack the severe, acute physical complications that result from Nbome.
Of course you should test all your drugs and not trust a new batch before getting it tested. And remember, kids: Just say NO to NbOme.