Trigger Assignment rule only when child org assignment happens

Summary:

Content (required):

Hi all,

we are having a requirement where assignment rule should trigger and assign a value to EFF only when child org assignment happens but not later in the stage.

we used below syntax

assignedToOrg("childOrg") as well as [Item].[Item_Basic].[ORGANIZATION_CODE] == "childOrg"

but both of the syntax triggers during child org assignment as well as in the update scenario post org assignment whenever updates happens in that child org. what should be an item rule syntax to ensure that item rule triggers only when child org assignment happens not later in the update stage.