In the British army, many of its component parts,
Corps and Regiments, have this 'Royal' appendage,
eg. Royal Corps of Signals, Royal Armoured Corps,
Royal Regiment of Wales etc, etc.
It seems that 'Royal' is a mere convention, with no
real significance. For instance, there were originally
6 Royal hospitals, founded by Henry V111, but today
there are many modern hospitals which have adopted
that 'Royal' soubriquet, with little, or no, justification,
eg. Sunderland Royal Hospital.