Manage Directs - Notify Old Manager

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hi All - I have a scenerio from the client and I need your help to see if its feasible or not. 

Manager A has a reports A1. Manager B has reports B1.

I login as A and assign B1 as the direct reports to A1.

In this scenerio, we want B to be notified through an approval rule. As per my understanding, the approvals will be triggered based on A1's heiarchy and not B1. Please can someone who has worked with a similar requirement confirm this. 

Thanks. 

Asha.

Asha