"RC" in this context means Research Chemical, these are substances that are usually novel and as such not yet well researched and regulated. They are called research chemicals because you're free to do your own "research on them" since there's no law stopping you from doing it. They often can be ordered from specialized vendors or from another country where manufacturing is done, like India or China.

These can either be actual novel substances in a given class or designer prodrugs that were invented to circumvent existing laws, for the latter 1p-LSD is an example, it's technically not LSD so it was legal but it turns into LSD in your body so it's not only mimicking the effect in another way like NBOMe, but it becomes the real thing. Anyhow, the downside with the other RC drugs is that there's usually little to no reliable scientific information about toxicity and other long term effects.

Benzos which this thread is about is a class of depressant drugs. But I guess you knew that already. I don't think they're very popular over here in Germany though.