This map that I attached has stacked units (non-building) on it...not sure how it's done. Just a little curious on how its done, any explanation?

http://www.staredit.net/wiki.php?article=Sprites#Practical_Use has a bit of information.

The only way I'm aware of is to place a unit sprite and mark it as Disabled in the sprite properties. Then, with triggers, you Enable doodad state on it and tell it to order. If you're aware of how to cloak units, it's the same trigger (except you don't need to disable it as it is disabled through the flags)... I thought there was a tutorial on it, but I can't find one.
Basically, for each preplaced disabled unit sprite do the following:
Enable Doodad State
Wait (0ms or do it with a death count timer)
Order (so they get a valid order instead of the invalid order they'd otherwise have)
Wait (same as above)
Move/whatever you want to do with them.
Repeat.

Thanks good info.