You may want to check how events work.
Contrary to a "usual" coding language, the picking (selecting) of instance is done by the conditions in the events and the actions/expressions apply to the picked instances.
In your example, when you click on a sprite, only this sprite is picked, and so it doesn't matter to refer to it through its UID or IID.
You might also want to check in the how do I FAQ (link in my signature) at the section "Picking/Selecting instances" there are several topics worth checking and explaining all about picking in details.