For das pass you need to go to guest relations at the start of your trip! It gets attached to your tickets and lasts for 14 days, so after 14 days just have to go back to guest relations to get reset! and to get a time for your ride you have to go to the fast pass queue and the cast member will scan your ticket and give you a return time, which is based on the current wait time, it then will show up on the app with fast passes
it can be used in addition to fast passes, but you cannot get another das return time until you have used the first
if you’re going to universal their policy is slightly different and you are given a physical paper pass that is signed at the attraction when you are given a wait time/ actually going on the ride, universal have also questioned me before (I’m anxiety sufferer) and it was a bit more invasive in that way, which wasn’t comfortable as other guests in the guest relations area! They instead tried to tell me which queues and rides would be unsuitable before they gave me the pass, I was there for a year and once I had one pass I had no problem renewing!