Particularly in the case of the old 1E AD&D modules, some of which were originally modules for what was then known as the AD&D Open, here's how it would have worked:
- The players are given a set of pregenerated PCs; they need to sort out which player gets which PC.
- The game is under a time limit (typically, 4 hours, which is the standard "time slot" for most gaming conventions). The goal is to accomplish as many of the module's goals as possible within that time limit.
- In the case of the AD&D Open, it's a three-round tournament. The top-scoring teams (from the standpoint of what they got done, and how long it took to do so) advance to the next round. (And, you advance as a table.)
As far as I know, the D&D Open (which runs at GenCon Indy) is now the only RPGA event that works this way. It used to be that the RPGA ran a lot of other "Classic" modules (i.e., pregenerated PCs), that weren't as "competitive" as the Open (these other Classics are often a lot more about role-playing, and a lot less about meeting goals; excessive role-playing in the Open tends to keep you from advancing).
However, the explosion in the popularity of the Living Campaigns, and then the D&D Campaigns, has pretty much killed the Classic format; other than the Open, I don't think that RPGA has released any other new Classic modules in a couple of years.