My thinking is this. All of the skellies serve the evil wizard. His main general is this guy:
since he's commanding the attack in King's Castle Siege (pretty comparable to the chief Nazgul in the siege of Minas Tirith). Next in line comes this guy:
I'd actually be tempted to make him the main general due to cooler armor, but the other one definitely has the main role in the Siege set, while this guy is off doing single combat. Under them we have the black and white skellies. White ones are the basic soldiers, and I see the black ones as either low level officers, or perhaps some sort of special ops soldiers.