Or sometimes the question is "is there a Servo To Go driver for Linux in there? If so where is it?". EMC's modular design is quite elegant, although it makes this particular question hard to answer. Are stg.c (for Model 1) and stg2.c (for Model 2) the drivers? No, these are modules which customize the driver for the Servo To Go card when they are linked in. They implement functions that the driver uses.
Can I point to the drivers? Yes, there's stgmod.o and stg2mod.o (and some variants) for RT-Linux. They are installable kernel modules. Communication with the driver is typically by a shared memory area (the other option is to use a fifo). The driver commands are listed in emcmot.h.