In the new expansion, does it matter which act you are in when you open a rift? Does it change anything if you open a rift in a different act?
Nephalem rifts are a randomized dungeons system. It doesn't matter where you open the rift or the difficuly in which you open it, the place of the rift and the number of level is decided at random.
The layouts stem from taking the tilesets and dungeons of the game and re-organizing/shuffling them. Monsters are likewise re-shuffled, found in combinations otherwise not present. The rifts have 1-10 levels. Each level is effectively a different dungeon, with a different monster combination. They are accessed via Nephalem Obelisks.