a grey area for what?

It’s not a ‘grey area’ to purchase TB with an Amex - it is an acceptable form of payment, regardless of credits. Amex has generally been crediting TB purchases (well…Amex hasn’t been crediting much this month at all, but in as far as they do, TB has been included for a while). Whether it is supposed to be within the scope of the UA incidental credit or not might be another question, many don’t believe it is supposed to be and so are suggesting folks don’t complain if it doesn’t. I purchased TB to use up the rest of the credit I had left at the end of the year last year, and credited fine. Other people who had their credits delayed have callled and asked about it - some people over on the Amex board seem to be pretty upset about it - but IME, it’s not really a big deal - Amex probably knows this is crediting but either the system for whatever reason can’t delineated between certain eligible credit items and this, and allowing it to continue is the solution they have chosen, or they know, want to fix it, but it’s not a priority and will get to it eventually. Whatever it is - go ahead and purchase TB to use your credit If you want.