This tells the game what the spell should cost, in terms of resources, when cast, such as an Action Point, Bonus Action or even a custom resource.
Example: data "UseCosts" "ActionPoint:1;Elemental_Essence:1"
In this example, to cast the spell, the player would need to have at least 1 default action point, and 1 custom resource, Elemental Essence, which will be expended on cast.