Negative Calorie Adjustments? What is that?

So I have set my Goal Calorie intake to 1400, adjusted my other values, and , set my activity to sedentary based on information from my dietitian.

I have turned on the step counter function in MFP to sync with my IPhone Steps (I'm waiting to get my new Fitbit Charge HR).I recently turned on the Negative calorie adjustment and I don't really get what its doing...Some days my steps count as calories burned and sometimes as calories taken away.

I guess I just need a basic understanding of its function...What are Negative Calorie Adjustments? What is supposed to do? Why should I use them? Will this work better once I have my Fibit and I can track all my actual daily activity?

Please Help..I just want my Calories eaten and burned to be as accurate as possible. Thanks!!