Originally Posted by gachen
How about in term of immigration? Does people need to clear any sort of immigration for a SJU flight to/from US? Is it just like any other domestic flight?
Correct. Just like a domestic flight. It is more like flying from MFE/LRD/ELP/IPL in that there are immigration officers around observing things, but there is no active official check that anyone goes through.
Note to prev post: STT is outside the US customs zone, while SJU is inside it. So, to travel STT-SJU, you would clear US customs at SJU.