RDA is a subset of RPA. While RPA means one can automated all types of applications on different platforms, Desktop, Web and mobile, RDA refers to an RPA that automates just desktop applications. Not sure why RDA became its own term these days. May be just another buzz word to put people off temporarily by throwing new buzz words at them.
IMHO, buzz words must be eradicated, especially when people create them without any specific significance and create them for the sake of creating.