Support Cards that have no active ability, but have great Auto-Abilities. They are the only non-Material Cards that do not have active abilities.
Most of these are available for purchase in the Item Shop for 2.000.000 Gil each.
These cards have extraordinarily high EXP capacity -- 2 million EXP at Lv.56, and over 5 million at Lv.80 -- making them a fantastic "EXP bank." High-end players (especially those with no better use for their Gil) have found that they can buy an Egg, fuse their excess Metal Cactuars to it, and then fuse that one Egg to any new Ability Card they obtain, for a one-step upgrade to Maximum Level without needing to hold large numbers of Cactuars in their Card Bank. Players can also load Egg cards in their Sub-job Ability slots, where the Eggs get a partial EXP gain from normal play, similar to cards you place in the Auto-Enhance menu. (It's recommended you use those slots for any other Ability you'll actually use in play first, though.)