When fitting multiple peaks curves, you can fit your data to a peak function by replicating the function into several "copies", each of which may have different parameters. You can customize these settings in the Settings tab, Advance page, Fit Control: Replica branch:
Use this feature if you believe that your data to contain multiple peaks of the same sort (e.g. Lorentzian) but different centers and/or widths. If the function you have selected does not support replications, this branch is disabled.
See also: the Multiple Peak Fit tool.