The FIB Stack Wizard step for background correction internally calls the module ‘Shading Correction Wizard' ; which itself relies on the module ‘Background Image’.
The ‘background’ is estimated, slice by slice, by fitting a 2nd order polynom on the grey values. It is able to take a mask into account for this fitting (i.e. not consider certain areas of the image, that may mislead the fitting process).
There are other modules suitable for background correction, such as ‘Background Detection Correction’ which is pretty flexible in terms of “definition” of the background.