Not sure about the FX, but on the Edge 2 manual- Overlap is the pixels it prints outside of the sent file boundaries- probably to cover the possibility of gaps:
"Changing the overlap setting:
1 Repeatedly press the SETUP key until the display reads OVERLAP
followed by the number of pixels. (The factory default is 15.)
2 Press either VINYL SLEW key to increase or decrease the amount of
overlap to a value between 5 and 25.
3 When your choice is displayed, press RESET to return offline."