An application which follows the persistence specification instructions.
For example, page 19 : Portable applications should not expect the order of lists to be maintained across persistence contexts unless the OrderBy construct is used and the modifications to the list observe the specified ordering. The order is not otherwise persistent.
If your application expects the list to be ordered, even without using OrderBy, then it's not portable. It might work on persistence provider A, but might not work on persistence provider B.