What is the difference between 6A-6B and 6A-6BX flanges?

Answer 1

6A-6B flanges are ring type joint which is not developed for face to face make up, where the bolting forces act on gasket.

6A-6BX flanges are also ring type joint but it is developed with raised face hence the bolting forces act on this raised face and not on the gasket if it is properly placed.

And also the raised prevent the flange damage from excessive bolting torque.