RTC = Roter Teppich Club = Red Carpet Club
I've been told the term came from the ubiquitous red on red decoration scheme commonly found at these clubs.
It's my impression that there are two ways to use the term. The tighter circle is a set of clubs that seem to be owned by the same people given the same girls will be found rotating through them. They also share common pricing (more or less), the default expectation (with the occasional exception that should self-identify) of AO services, and other little things like using the same products.
But there is a wider circle that doesn't necessarily imply common owners, merely a similar business model. I suspect that La Luna is outside of the tighter commonly owned RTC circle. Less clear to me is Dietzenbach which has a similar business model, but doesn't seem to use the same pool of girls.
This is, however, all based on observation and inference. I've yet to speak to someone who has certainty about who owns the clubs. Perhaps under the new laws one could find out by accessing the licensing records.
RTC = Really Tiny Coke? (joking)
I've made this joke because one of the common products at the RTCs are these almost toy-like self-serve coke machines that fill small glasses about 3/4 of the way.
I've heard a similar joke that's much less charitable to the customers.
(Dietzenbach also uses one of these tiny coke machines. Does that indicate common ownership with other RTC's, or did they just copy it? Dunno.).