I'm likely not understanding. Extents, in my mind, are the extents of the machine travel. Your X,Y,Z zeros on the stock corner can be anywhere you set it. And once set, hopefully your gcode travels stay inside the extents! With the stock Mach4 screen colors, your extents show on the display in grey dashed lines, I guess, really, IF you have it turned on to display, your gcode travels show in colors for feed and rapid moves. If you close the gcode, load another, the new one shows in colors. If no gcode is loaded, you can still see the extents, defined in Homing and Limits, on the screen, showing the spindle location, and it will always be the same, no gcode, gcode, doesn't matter to the extents.
If you have limit switches, they will stop you from going outside the extents, if no limit switches, soft limits can stop this. You should have both, but for sure, soft limits set to the "extents" of you machines travel. Physical limit switches are a last resort, personally in 10 years with my mill, I've never hit one. I have hand cranked up to the soft limit, however:) I have soft limits set .020" off the hard, physical, last-resort, limit switches.
But I digress.................