Flanking allows you to bypass the benefits of Cover. Flanking can be done by going around the side, or even going above or below the enemy in Cover.
Partial Flank
A partial flank occurs when you have changed your position to the point where part, but not all, of the enemy is revealed. A partial flank reduced the enemies Full Cover to Half Cover. What limbs become uncovered by a partial flank is up to the GM.
Example
You are attacking a Raider hiding behind a wall, which is considered Full Cover. You run around the side, to the point where you have a clear view of their Head, Torso, Left Arm, and Left Leg. They will now only gain the benefits of Half Cover against you.
Full Flank
A full flank occurs when you have changed your position and completely revealed the enemy. Cover no longer provides any benefit for an enemy which is fully flanked.