Super Sea Snails can be used to add slots to gear that can use an extra slot. The max is four: One main ability, three extra abilities. I'm sure you already knew that though. You can only use a super sea snail on a gear to add a slot ONLY when you finished leveling up the gear. If you use one on a gear with four full slots, it rerolls the three extra abilities. I've mainly wasted mine on rerolling