how an item is picked if a picking rule is not there

If a picking rule is not available at the time of pick release, how does the system know which item to pick,what priority to choose etc.
we have a system where the contract manufacturer dumps the finished goods into a location and later a 3PL comes and picks up the material and ship confirmation happens. so just wondering how /which item will be picked up by the 3PL if a picking rule does not exist.
Thanks.