Mac got most of it
But there are 2 different assistance services at Universal
what Mac described is AAP (Attractions Assistance Pass)
and it does indeed get you immediatly through the express line if the wait time is less than 30 minutes (but the TM have the final say. They may not grant immediate access even if it's under 30 minutes, depending on operationnal considerations)
if the wait time is over 30 minutes, you'll be given a return time based on the current standby time, minus a fraction of that time.
AAP is designed for guests with non visible disabilities. You'll need to discuss your issues with a TM at guest relations
it's valid for 14 consecutive days.
The GAP (Guest Assistance Pass) is a whole different story.
It comes in very, very limited amounts, and is very hard to get. It's designed for those for whom the AAP will not be sufficient.
the GAP is basically a 1 day/2 parks unlimited express pass. It will work like the express pass you can purchase or get by being a deluxe onsite hotel guest.
there are 3 main limitations
- it's very hard to get
- it needs to be renewed every day (see point above re: hard to get)
- it works like an express pass, meaning this won"t get you front of the line (aka express line) for the main Harry Potter rides. (those that don't have an express lines)