It turns out you can't create a where clause in Pivot.
But, you can create a calculated field. In the Data Model Editor, Add Attribute -> Eval Expression. I used the field name is_internal_communication , and an evaluation
if (sender_domain = receiver_domain, 1, 0)
After previewing and saving, I was able to create a new child object with the constraint is_internal_communication = 1 , and obviously I could add a child object with the constraint in_internal_communication = 0 for external communications.
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