It is implemented as a second order polynomial in the region, so constant, slanting or curved (parabolic) background may be modeled with it, freely combining its counterparts. As a background function, its area will not be summed up to the peak area. It may have at most three fitted parameters: Level, Slope and Curvature.
The picture shows a sample polynomial background with Level=10, Slope=0.5, Curve=-0.03 parameters.
Channel position within the fitted region (first channel of the region is 0)
Constant background level
Background slope (may be positive or negative)
Background curvature (may be positive or negative)