You need to look a bit on picking and conditions
If you dont have any picked instances the action will work on all instances. But some events picks one or more instances. The following action will then only affect the picked instances.
For example if your event condition is "is X overlapping with Y : destroy X"
will only destroy instances of X that are actually overlapping with a instance of Y
(if you set the pick options correctly)
in pseudo code it is will behave a bit like this.
for each pair (x, y) where x is an instance of X and y is an instance of Y.
if x and y is overlapping: