I was once told XYZ are generator terminals, UVW are motor terminals (the other set of terminals are usually U1, V1, W1) and RST are terminals of a receptacle and other stuff in/ on the walls.
The motor designations have stuck, receptacles, panels and everyting else now have the standardized L1, L2 and L3 all across Europe.
Phase colors way back were black, red and blue too! That is if there was no ground wire. In that case the phases would have been black, blue and grey. Unless there was a neutral... then the phases were black, blue and black.
Guess why that color scheme was abandoned in the 1960s...
Side note: in reality at least in Austria all those colors were hardly ever used. Conduit wiring mostly had all 3 phases the same color, for example 3 blue wires and a red one for the ground.
