Fill in my knowledge gap :)

[A_Camera]

Do you mean -Z50?

No, he means Z+50.

His reference for Z0 is the base of whatever holds the work piece, i.e. the bottom of the work piece. First time surfacing only, after that turning the work piece around and referencing Z0 to the surfaced side (now the bottom of the work piece) and lifting the Z to Z+50 to get the work piece exactly 50 wide. If he needs to cut far too much then he lifts Z over 50 and uses as many passes as necessary to arrive and stop at Z+50. At least if he is doing it the way I do, and the way I interpreted his words.