Well, the short story is that we have a minimum amount of hours that we've got to be paid during any given day called "variable minimum".
I think, and any other Delta guy feel free to correct me, that we've got a "minimum" of four hours of pay per day on the trip. Basically, that's there so we don't fly 1 hour from Billings to Salt Lake City. Sit for 7 hours in uniform at the airport, and then fly Salt Lake City to Denver.
It's "supposed" to help from being on duty for 14 hours in a day and only winding up with 2 hours of pay.
Without that, you run the risk of having (theoretically) a four day, ten hour trip where you'd have to work 25 days to make a honest month's work.
Basically, if the trip doesn't reach the days "VM", we'll have "credit time" to bring us up to it.