Different types of relationships a user can have with a grant.

CALLER_RELATIONSHIP_TYPE_UNSPECIFIED Unspecified caller relationship type.
HAD_CREATED The user created this grant by calling CreateGrant earlier.
CAN_APPROVE The user is an approver for the entitlement that this grant is parented under and can currently approve/deny it.
HAD_APPROVED The caller had successfully approved/denied this grant earlier.