A VERY useful class. See UGameplayTagReponseTable. You set it via GameplayTagResponseTableName in UGameplayAbilitiesDeveloperSettings.

/**
A data driven table for applying gameplay effects based on tag count. 
This allows designers to map a "tag count" -> "response Gameplay Effect" relationship.
For example, "for every count of "Status.Haste" I get 1 level of GE_Response_Haste. This class facilitates building this table and automatically registering and responding to tag events on the ability system component. 
*/