Express can be very confusing...
As you already know RPR, HRH, and PBH room keys get unlimited all-day express with their room key.
If you're not on-site, express can be obtained in two ways:
(1)Free Express. This is the standard express pass. Anybody can get it, for all you need is your admission ticket/AP. You scan the mag stripe or barcode at the express machines and you choose what attraction (if more than one is available) and what time you want to return (if there are enough time slots). You'll then get a white, printed ticket with your return time. You can only get one pass per ticket, unless a special promotion is running. You can be up to 5 minutes early or as late as 30 minutes after the hour window. These express aren't always available at park open. Some dont open till 10. All express machines close when: (A)The maximum amount of tickets have been distributed for the day. (B)It's 2-3 hours before the park closes. (C) There's bad weather. With these express, you can get one every 1-2 hours.
(2)Paid Express.These express passes are purchased for a price (anywhere from $10-$25) and are VERY specific about their use/limitations. Each person in your party must have a pass. Passes are printed onto a ticket which looks exactly like a park ticket, but it says "ISLANDS EXPRESS" "STUDIO EXPRESS" or possibly "COMBO" on it. The "COMBO" one is the only one valid at both parks. Well, that and the "BONUS PLUS EXPRESS" which is also available. Each pass is good for ONE express admission per attraction per day. Each pass is dated. If you try to re-use it, the scanners will show that it's already been used. Plus many places mark that you've been there already, so they know they can't re-admit. It says in the fine print that the ticket is good only one per attraction and that it is only valid in the park it was issued for.
Funny story: Today, I was calmly explaining to this lady that her Disney Assitance pass isn't valid and she got all mad at me! I explained to her that the GKTW pin her daughter had was fine, but the Disney pass (I even pointed it out to her) was no good. I did laugh it off later.