Guess you've been looking at UK/European milair lists?
TAD = Tactical Air Designator (this isn't strictly correct but is close enough). Freqs are given a number and this number is used rather than passing the freq openly over the air. There are NATO common TADS and there are TADs used by individual airarms/navies
STUD = European equivalent of the USAF 'PRESET' ..ie a memory channel on their radios much like those found on car radios etc.