G28 is return to first reference return position. I usually use G0G28U0W0 on a lathe. Seen some horrific crashes from people not specifying an incremental move like G91 or the U and W. In fact, at my old job, a applications engineer was teaching a customer how to run his BRAND NEW Mori horizontal. Hadn't even cut a chip yet. Well, it was look how fast this thing is. Boy was it, he forgot the G91 command with the G28, X and Y bounced right off the pallet. Bent the spindle, destroyed the X axis linear guides. At that kind of rapid speed, things to go very wrong with steel to steel contact.