When ordering books online I see 2 terms frequently with regards to paperbacks. B Format and Trade can anyone explain?

I’m not sure about B format, but trade I believe is 6×9 inches. That’s the most common mass market size in the states. (I think but will happily be corrected).

When I was trad published in the UK my publisher told me my books were trade size and they were 8.5 x 5.5inches. I still make my books that size because I wanted them to look the same size as the ones previously published. Maybe it’s different in the US.