By changing the analog calibration, you can customize how your SyRen/Sabertooth responds to analog input. The modes are:
0V is full reverse, 5V is full forward, and 2.5V stops.
The range is whatever you specify. One possible application is single-direction movement: set both minimum (full reverse) and center (stop) at 0V, and maximum (full forward) at 5V.
The center is whatever the analog input reads at startup. Full reverse and full forward are determined by the minimum and maximum values seen, respectively. This is useful in cases where the center varies quite a bit from 2.5V but can't be counted on to be the same value all the time. Joysticks sometimes fall into this category.