I think the correct item to use is:

or:OrderResponse/cac:OrderLine/cac:LineItem/cbc:LineStatusCode

As there is no recommended code list member for backorder, you would divine one for your community of users.  Then for validation purposes there are a number of steps to take to replace the value validation with the appropriate XSLT.

But before doing so, you should get another opinion that what I've guessed is correct or not.

Because this forum discussion area is not active, your question is better asked on the UBL Dev mail list ... please review the instructions at the top of http://ubl.xml.org/forum for details on how to register yourself on the mail list.  There are experts following the mail list who can help direct you.

I follow that list myself, so if someone confirms my supposition, I can follow up with details on the steps involved to change the validation.  Of course you don't have to modify the validation if you are leaving validation up to your application, but typically one would modify the validation so that the application doesn't have the responsibility to check values.

Thank you for posting!

. . . . . . . . . . Ken