Action and conditions do an internal for each over each picked instance of the object type the Action or Condition is on. (i.e. system is only one instance, so using the create object system action will only ever create one object, while Sprite spawn object will spawn multiple objects if multiple Sprites are picked).
There are outliers like system picking conditions, which again do an internal for each over picked instances of the object type selected in that picking condition.
If you add an event sheet for each the whole event is repeated for each picked instance, with only one of those instances picked.
(this may be a bit of a simplification of the actual engine internals, but thinking about it this way holds true)