Single Segment is the same as RTA, except you're not allowed to reset the game or load a save.

^ exactly that.

As a side note - Technically anything that uses RTA timing (single segment or otherwise) doesn’t have to be done in one sitting. It simply means don’t pause or stop the timer unless the run is over or you’re resetting. For stupid long runs (cough cough Baten Kaitos 100%), it’s not possible to do it one sitting. However, they still use RTA timing because they don’t stop the timer while they sleep. Animal Crossing 100% did this too back when the run was like 70 hours.