The way I understand it, if she goes to Preference at both, it will depend on where she lands on the groups' bid lists. If she ranks XYZ #1 and she is high on their bid list, she will get a bid there, regardless of whether ABC wanted to bid her. However, if she is on XYZ list but they fill their quota before where she is on their list, and she is higher on ABC list (which is likely if she is a legacy), she may end up with a bid at ABC (if she ranks them on her bid card).
(did I get that right, more knowledgeable-PH-type-people?
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