Fitbit will measure all your steps throughout the day, however if you want more visibility for certain activities, for example if you wanted to see how many steps you took/how far you went when you go for a run/use the treadmill etc. you can log it as an activity in fitbit, it doesn't change the total step counts or calorie counts but it means you have more data to look back on. You can also log activities where it shouldn't be logging any steps at all... for example driving on a bumpy road. Or you can use it for activities that the fitbit generally doesn't track well like riding a bike. I would advise setting you MFP profile to sedentary, that way on days you don't move much your calorie allowance reflects that as the fitbit won't give you more calories on those days, but on days when you're more active the fitbit will give you a calorie adjustment to reflect you activity level.