Express vs Priority
Express and Priority are two different systems that may serve similar purposes but are different.
Priority Queue
Priority is a fairly simple system, where players get a priority of 0 by default, and any players with a higher priority will get put in front of players with a lower priority.
The downside to Priority Queue is that theoretically it is possible that if enough paid/priority players keep joining the queue, no standard/non-priority players will ever be sent.
Express Queues
Express queues are an ajQueuePlus feature that is meant to be a more fair version of priority queues for paid players.
Instead of putting players in front of other players, the Express Queue is a completely separate queue from the standard queue. The idea is that since only paid players will have express queue, it will be a shorter queue.
ajQueue alternates between sending players from the express queue and standard queue, so that non-paid members will not get cut in front of constantly like with priority.
By default, ajQueue sends one from standard, then one from express, and repeats, but you can change that using express-ratio in the config.
For example, you could have it send 2 standard queue players and 1 express queue player.
Priority and Express together
If you want, you can use Express queues and Priority Queue together.
If a player has the express permission, they will always be sent to the express queue. If they also have priority, then they will be put in front of players with lower priority in that queue.