can be used at any time, to return the number of picked instances of a sprite.
eg. A overlaps B >> if none overlap, then it will return 0, since none are picked.
You can use it in nested structures of picking subevents to check for the count of picked objects and it would return different numbers at different points of the tree.
i know you can do this with a for each and a variable, but having an expression could come in handy.