G54,G55,G56,G57,G58,G59 are work coordinates, lets say you mill a dice on rotary axis trunnion table, you can make 5 of the 6 faces a work coordinate. In the industry its known as a 'wpc' work piece coordinate. G53 would cancel any of those wpc, infact i recall you should cancel before calling another wpc. G17, G18 and G19 should be placed before the wpc code if desired, this enables tapping, drilling, boring, etc to be used in a different plane. A good example is a huron milling head, if you rotate 90 degrees in any direction you can then used 'canned cycles' such as drilling in your new plane. Always remember to apply a cancelling code after enabling otherwise the machines 'ready state' will have a different plane or wpc active, a crash will occur when restarted.