RPT ia a classification given by the Piano Technicians Guild or PTG (got to  PTG.org  to find out more).  An RPT has passed a series of tests at at least a minimum competency level for proficiency in aural tuning, action repair and regulation and various other issues like string splicing and replacement and lyre adjustment and repair. These tests are fairly rigorous and require at least a couple years of focused study and experience to pass.  Not all professional technicians choose to take these tests - even some members of the guild, and not having the RPT designation does not necessarily infer lack of experience or skill.  Guild members have an Associate designation until they pass the tests.  

PTG members are encouraged to become certified as RPTs in order to ensure that members are providing competent service with a basic knowledge of the depth and breadth of the field of piano technology.  Members are also obliged to follow a set of professional and moral codes of conduct laid out in the bylaws.  As a professional piano tech I consider the yearly fee to be well worth the value I get from information, extra training, and the peace of mind it gives my more informed clients that I consider myself a skilled professional and act accordingly.