I'm sure that many of you know this, but I did not. It seems that TB funds cannot be used for Comfort+ seats. I had $1500 in TB and tried to book two C+ tickets that were $1288 with taxes. But booking system would only apply TB to cover the cost of the economy seat and taxes, not the amount stated for C+ (~$360). The t&c didn't seem to preclude using TB for C+, at least when purchasing the tickets and not as a later upgrade, and this was a UA flight. I called an agent, and they confirmed that TB cannot be used on C+ because that is considered a seat selection fee. So, lesson learned. I will stick with using TB for first class tickets in the future (I'm quite tall, so need the extra room), which TB fully covers.