Uncommon when used in Sos battles refers to when a Pokemon can call something other than itself. This usually happens less of the time than if the Pokemon calls one of its own species, (in your example, Corsola will call another Corsola more often than it will call Mareanie.)
Most of these Pokemon are evolutions of the caller, like Dragonair/Dragonite, Butterfree, Braviary/Mandibuzz, etc.
