This alumina spectrum has a region where HyperLab fitted a background curve (2nd order member of the background polynomial), and completely eliminated the slope (1st order member). If you are curious if the fitted curve is necessary, or it may be replaced by a slope, follow the steps below:
When you manually modify a parameter of the fitted region (and change to another editor field), HyperLab automatically displays the new model with a different color scheme. This way you can immediately check the new model. This is, however, only a visual representation, and the entered data is not used elsewhere until you click one of the Set or Set & fit buttons.
As a new fit is required now, the Set & fit button should be clicked, therefore the manually set values are used only as an initial estimate for the fit.
After the fit, the new model is displayed with a background slope instead of a curve. Chi-squared changed from 1.8 to 4.3, so in this case the experiment showed that the slope is a worse parameter to fit the background than the curve.
Click Undo button to restore the previous fit now.