It's a parameter used by one of the postprocessors. Basically, I think that Marlin 3D printer used as CNC, don't know about G0 command (move as fast as possbile command). Actually using G0 is the same as using G1 in this specific case. So you have to use linear move command G1 (or G0 since it's the same) set with a very fast speed that the printer is capable in order to get the effect that G0 is usually have. This is a hack and the parameter is usable only for this special case. For other cases, ignore it.
Ok this seems to be the case. Its used for G0 moves
It's a modification that I've done a while back on a user request but since I'm not a Marlin user, it's not something I keep track of. I may add a tooltip info to specify that this parameter is used only together with Marlin postprocessor file.