Do you mean the large green button on the PMDX-179-PAN (with the panel)? That was originally meant to be wired into the 2-pin screw terminal on the PMDX-179 when the PMDX-179 was connected to a PMDX-126. In this configuration the "Reset" button (or "Test" push--button on the 179 board itself) performs the same functions as the "Test" button on the 126. For example it causes the 126 to re-read the DIP switches, and also clear errors when charge pump is disabled.
When the PMDX-179 is connected to a PMDX-424, the "Test" button input on the PMDX-179 is not currently used*. There is no PMDX-424 equivalent of the 126 "charge pump disabled" mode, and there are no configuration DIP switches on the PMDX-424 to read. You may, however, wire the "Reset" button directly to the PMDX-424. You could then program Mach4 to read that as "feed hold" or some other function.
* future plug-in/firmware releases *may* add uses for the switch input (and DIP switches) on the PMDX-179.
Bob