It's pipe thread, Rc is a female thread and is conical. It will form a pressure tight fit with a cilindrical R male thread.
Standards:
Tapered Whitworth Pipe Threads BS 21, ISO 7/1
ISO R Designed to mate with threads
conforming to ISO Standard 7/1, a tapered
pipe thread available in male form. ISO R
ports are similar to NPTF threads, they form
a pressure tight joint via an interference fit
as they are tightened, (thread sealant is
recommended).
In addition to itself, the ISO R thread form
will also mate with British Standard Pipe
Tapered (BSPT) and Japanese Industrial
Standard Tapered Pipe (PT) per JIS B
0203. Some form of thread sealant must be
applied to the thread to prevent spiral
leakage.
ISO R male threads will also effect a seal
with ISO G, BSPP and JIS B 0202. In all
cases, some form of thread sealant must
be applied to the thread to prevent spiral
leakage.
