The PX field Account -> Parent Group ID is a field incorporated in the PX Account model for the purpose of linking multiple PX accounts to a single company record in CS. While this field is available in Audience Explorer filtering, it is not available in Account attributes in Engagement audience logic or in Segment rules.
The customer use case involves using complex filtering to isolate 500 Accounts that, would Account -> Parent Group ID be included in the Account attributes in Engagement audience logic, could be replaced by a single rule.