Hum there is a mistake in the explanation right before.
The UID doesn't "slide"/change. If you delete an instance, its UID is simply gone. The IID is the value getting rearranged on spawn and destroy of instances.A quick capx to demonstrate
You can see in the capx, that on the second sprite line, there is an offset, we'd expect UID 4 and instead the first instance on the left is 5.
That's because I added the mouse object before deciding to add the second line.
You can also see if you right click a sprite to destroy it that the IID gets reorganised for the instances that are left, whereas the UID stand still.
And the "tutorial" you were originaly looking for should be listed in the how do I FAQ
, section "Picking/selecting instance(s)".Kyatric2012-05-10 02:18:06