I couldn't remember so I had a look at the code.
It can't summon any kings, and no "glass" units from the puzzle mode. And no "misc" units which is mostly blocks and stuff.
It also additionally can't summon:
Kristian Maria's summoning circles, Soft Swarm's egg, gogo stone, sacrifice, Bonus Man's cornucopia or something called "Terran Warp" (I forgot what that is)
These are the same things that the opponent cannot summon in Endless.
So the spell can summon the unique Endless units. Some of those are pretty good/weird.