Originally Posted by ijgordon

It's really a function of your corporate travel policy.

There are a couple of issues. I'm of the view that these YUPs/KUPs are just discounted business/first class fares. If your receipt shows the booking class but not the fare basis, then your expense approver will see "A" or "P" which is First Class, not "Y" or "K".
Additionally, if you book, say, a KUP fare (into P inventory), there will almost always be a K coach fare with similar restrictions (e.g., refundable) for less money. Unless K inventory is completely sold out, then by purchasing the KUP fare, you are explicitly paying a higher fare than necessary for a confirmed first class seat. That sounds to me like buying a first class seat, which would be against policy. Same issue with YUP (A) -- there's almost always a "Y" or "H" fare for less money with similar rules.

If what I'm saying isn't clear, try an example: LGA-MIA -- KUP base fare is $444 o/w (fare basis KUPP26T). However, there is a K coach fare (fare basis K26ZT) for only $304 o/w. Both are refundable. I don't know how anyone could justify buying the KUP fare if the travel policy requires coach travel, assuming the K inventory is available.

All this being said, some corporate travel departments apparently are fooled by this YUP chicanery.