I'm not qualified to answer that...

even though my band did open for several top groups. in OUR case, it was never the headliner's call... BUT... no doubt at all that it would be for the most popular groups.

Which brings up.... Santana used to come to Fresno often, (near me), and we played on the bill with them often... but NOT just before them. There would usually be 3 or 4 groups, we would be 1st or 2nd. Never the less:

We were backstage with the headliners... both before and after the concerts. We did a lot of partying back there.