Hmmm... that helps with some UP fares that book into A such as this:
FARE CLS EXPLANATION BOOK CODES
-------- ---------------------- ----------
QA07UPNP ONE WAY INSTANT PURCHASE FARES Q/A/I

Although, DL automagically takes some standard fares and tacks on the WNUP and they book into P even though the base fare rules without the WNUP don't say P is in the booking codes. Example:
For BOS-DFW KR07A0UP says booking code is K, but DL will sell it as an A for 50% more with the /WNUP on the end and then the booking codes are K/A/I.