Control is the FPGA control/indicator that you wish to write to (consider you have more than one 8 bit control, you need to tell it which one). The session is a way for the driver to know which FPGA bitfile you're talking about (and, in the case of systems that support more than one FPGA, which FPGA).
The NiMyRio19XX.h that matches your controller has the control constants in the header to control where the reads/writes go.
