The buttonband allowance is the space between the two fronts on a cardigan, not including the edging.
Entering 0"/cm for the buttonband allowance will result in cardigan fronts that meet in the middle before adding trim. Entering 2"/cm will result in cardigan fronts that are 2"/cm apart before adding trim. Entering -1.5"/cm will result in cardigan fronts that overlap by 1.5"/cm before adding trim.
If you would like a standard buttonband that exactly covers the gap between fronts, enter the same number for the allowance and the buttonband edging height.