RF leg
RNAV procedures relay on FMS databases. These databases are coded with path-terminators according to the Arinc 424 standard. There are 23 of them like, CF Course to Fix, IF Initial Fix,CF Course to Fix or RF Constant Radius Arc Leg.
Not all of these path terminators are required to construct a RNAV procedure,that is, a PBN specification, and not all of these PBN specifications require the same path-terminators.RNP AR APCH and Advanced RNP are the only specifications where RF is compulsory but the rest of them like RNAV 1 or RNP 1 (used for SIDs and STARS) normally don't use it.
RF permits to fly an arc very accurately, normally based on a database fix, opposite to the "traditional arcs" where the plane just flies from one fix to the next one, not following always the same track.
The RF leg is designed to be used for terminal procedures.