The clean_regress_combine control parameter was modified in April 2018 to have five choices (previously it was an on/off switch). The options now are
disable - no regression, residual map added to component map or not, according to media mode (this is the default)
full_resid - Regression of the component map expected counts against the observed. Scale the component map by the regression coefficient and add the unscaled residual map
scaled_resid - First do the steps for 'full_resid' but then take the counts minus the scaled expected counts and regress that against the expected counts from the residual map. Form the new map from the scaled component map added to the newly scaled residual map
old_scaled_resid - Method prior to April 2013. Regression on the expected counts from the component and residual maps.
no_resid - Perform the regression of 'full_resid', scale the component map by the regression coefficient and do not add any of the residual map, a media mode for regression.
These options are implemented in the routine hsi_clean_regress_combine.pro if you want to take a closer look.