What Values will DiplomaticYieldSource take?

[Magil]

For the MODIFIER_PLAYER_ADD_DIPLOMATIC_YIELD_MODIFIER Effect, I was curious as to what you could use as triggers in DiplomaticYieldSource. John Curtain uses LIBERATION and WAR_DECLARATION_RECEIVED, and Cyrus uses SURPRISE_WAR_INITIATED. I tried to use JOINT_WAR_INITIATED in my own modifier but it didn't seem to take, though I didn't receive any Warnings or Errors in the logs (it seemed to accept it as a valid modifier, it just didn't trigger upon declaring a Joint War). Anyone know what you can input as a Value for this Effect?