The web can enlighten you a bit more as to the nature of it.
But essentially it is the Machine (M,S,T code) control logic file, it is a boolean representation of what originally used to be external hard wiring to control the machine I/O, as some early CNC were wired this way.
Depending on what make of CNC controller you have will depend on how it is implemented.
In the early days, when machines went to PLC from hard wiring, the PLC was an external unit that could be implemented by virtually any third party supplier, now most CNC have their own co-processor that works hand in hand with the CNC (motion) controller and each pass info back and forth to each other.
The bottom line is, it is often a separate file that has to be loaded with a specialized PLC file loader.
If yours H.S. is an older Fanuc it is usually done this way, Mitsubishi can be loaded from RS232 etc, or edited on screen.
Does yours have the Seicos Control?