Block hours=hours you actually fly. Usually from the time the door closes to the time the door opens. This is what is used for legality purposes as well as what you can put in your logbook. Credit is what you get paid. So, if the gaurunteed credit is 43 minutes for the flight and you fly faster, say 39 minutes, you still get 43 minutes of pay (credit pay) but log 39 minutes of flight time (block time).