Grid Azimuth/Range Spacing Only determine how many GCP will be calculated.
Registration Window is Centered at one GCP cell. The slave and master registration windows are directly used to match for best coherence,without the involvement of a reference patch moving step-by-step.it can returns the final negative or positive range and azimuth offset for once. all in all, its done by some math tricks.
Finally, there is a Oversamping Factor used to get sub-pixel accuracy offset or velocity. of course also achieved by some math tricks.
if you want to get into the math tricks box, turn to( Question about Normalized Cross Correlation)