What make a Volks one off a one off? Is it a specific head mold that's only used one time or is it also a character, makeup and or outfit? I noticed searching that the head plates will say one off. Bobby
I believe it's called a One-Off due to the faceup and outfit being unique to the set, and not so much about the sculpt itself (as they can re-issue the same sculpt for a different One-Off fullset).
What @satyrsmoon said, plus Volks sometimes use sold-out limited heads for one-offs, or combine a usually-male head with a female body (or vice-versa). I think they might occasionally make a one-off in a different resin color from the one usually associated with that particular headmold -- but I'm not as sure about that as I am about the other types of one-offs.
What the others have said. From what I've seen, the One-Offs are made as uniquely painted and dressed dolls that are put on display at the Volks stores for a set time. Customers can put their name in a draw box if they want a chance to purchase that specific dressed doll. Unless the sculpt is limited, the One-Off Models don't have any greater marketplace value than a standard release. The head-plates ID the origin of the doll (location/event and manufacture date). A One-Off plate lets you know that the doll has been previously displayed and the head/body combination might not be the same as a standard model (swarico legs, option hand parts, etc).