During each Extra Ops event, typically one or more units are added to the game which make use of items only they may equip. These items are, for the most part, readily obtainable by playing stages specific to that event - but they carry extreme fusion costs if trying to build them outside of it, assuming they're still available at all. In at least one case an older event item has been required by a later event unit (eg Kraken from Black Future requires the Treasure Box from Mummy's), suggesting that the serious collector may find it worth stockpiling surplus equipment.