I've pointed this out elsewhere but the "Mystery Amount" in Formations is how the game sees what enemies to summon using Call or Alert from an enemies attack. 0 means that no monster will be summoned and monster type 2 will be displayed normally. 1 means that 1 will be summoned and monster type 2 will only appear when summoned. 2 means that if there are 2 type 2 monsters both will be summoned, if not 1 will be summoned. And I couldn't find much of a difference with 3, but it does not allow three monsters to be summoned.
I've also pointed this out elsewhere, but the "Underground" option in the Map Info. is actually something which means "Use Ending NPC Indexes"