When asking for flight following you need to give your N-Number, type of aircraft, destination, and initial altitude. These codes are the old domestic IFR flight plans. They are intended for IFR navigation so they assume that the aircraft has at least one VOR receiver.
IFR flights now require FAA Flight Plan Form 7233−4 which has more detailed equipment codes. Codes for the new ICAO compliant flight plans can be found in the AIM TBL 5−1−4 and TBL 5−1−5. If you use an EFB, like ForeFlight or WingsX, it can guide you through filing with the right code.
RVSM stands for Reduced Vertical Separation Minimums. RVSM airspace is where air traffic control separates aircraft by a minimum of 1,000 feet vertically between flight level (FL) 290 and FL 410 inclusive. Global Navigation Satellite System (GNSS) is a generic term for satellite-based navigation systems like GPS/WAAS, Russia’s GLONASS, and Europe’s Galileo systems.
My Cherokee has a Mode C transponder and one VOR so it is a /U
An Arrow I fly has a non-WAAS Garmin 530 so it is /G.
My Cessna 210 has a WAAS Garmin 430 and it is also /G.