It doesn't change during runtime, but you cannot count on what it will be until after the object has been created at runtime. Each time you run the game the UID will likely be different.
Think of it like this - if on preview 1, you create an instance of a sprite, it will have a UID, like 1. If on playthrough 2 you create an instance of sprite2 first, then create a sprite instance, sprite will have a different UID, like 2, than it had on the first playthrough, because sprite2 was assigned the UID 1 first because it was created first.
Does that make sense?