Fitbit tracking walking but does it sync classes like spin or should you add these?

Your fitbit adjustment is the number of calories your fitbit says your burned all day (including any exercise you tracked on it) minus whatever calories mfp thinks you burned (including any exercise you logged on mfp).

If you recorded the spin class on your fitbit, it’s already accounted for and you’ll get the calorie credit for it. It will not show up in your diary as a specific workout though.

If you log the spin class on mfp, that amount will overwrite what fitbit recorded for that time period. So logging it on mfp will just replace the fitbit entry that already exists.

As long as you record exercise on your fitbit, then you don’t need to log it on mfp.

If you don’t record exercise on your fitbit (like if you have the One model), then you’ll want to log non-step based exercise on mfp.