Elemental Bursts are the ultimate refinable abilities a character can have in combat. They require and cost a set amount of elemental energy to activate and cast but are more powerful than a standard elemental ability. You can only be attuned to one type of elemental burst at a time. Each has its own quirks and nuances that should be studied in combination with other abilities to best know how to use them in a team setting!
The elemental skill & elemental burst categories represent the 3rd "core set" of assets involved in the cortex dice roll. They control the core gameplay mechanics of fights by focusing on applying and using elemental energy. Its counterparts are "utility passives" which typically only apply in non-combat scenarios. You may only have one skill or burst at a time.