Per this post in the forum: https://developer.sailpoint.com/discuss/t/combining-event-trigger-filters-with-attribute-filters/2183
It would be useful to be able to filter events based on attributes not present in the payload, but related to the resource in question. For example, it could be useful to filter Identity Attribute Changed events based on the identity's manager, which doesn't appear in the event payload unless it was changed. For source created events, it could be useful to filter events based on additional attributes of the actor that created the source. Maybe we want to only receive source created events if the actor that created it is in the "Engineering" department.