What's the Difference Between Second Unit and Action Unit?

I've never really done second units as the first a d before, until the marvel thing. It's just kind of almost like a back up unit also. I know paul likes to call it action unit, which kind of makes more sense as second unit. We're also another first unit at times. Each show is a little bit different. The ones that we've done are not all marvel s have a full time second unit. But i've found that there's always the knowledge that there's going o be a whole lot more coming to us other than what's figured out from the beginning. So i think that we're there.