G54 you use to say “Use Coordinate System 1”
G10 L20 P1 X0 Y0 Z0 says “For coordinate system 1, set the current position as the (0,0,0)”
G10 L2 P1 X5 Y5 Z5 says “set the global position of (5,5,5) position as the (0,0,0) for coordinate system 1”
At least that’s how my 5 minutes of research has made me understand it… #stillanoob