Mina,
Try something like this:
Substitute your ZBARCODE where I used description. The above rule will check each record in the EquipmentWorkList collection, looking for an Equipment Description that matches the current transaction's Description property. The Equipment fully qualified object name value is necessary so Agentry knows to what context our variables belong. If we do not find one, the validation rule will pass. I did not test this, but believe it should work for you.
Jason Latko - Senior Product Developer at SAP