Multi-parameter functions are used as built-in functions for Origin's nonlinear fitter. You can view the equation, a sample curve, and the function details for each multi-parameter function by opening the NLFit (Analysis:Fitting:Nonlinear Curve Fit). Then select the function of interest from the Function selection page.
For additional documentation on all the multi-parameter functions available from Origin's nonlinear curve fit, see this PDF on the OriginLab website. This document includes the mathematical description, a sample curve, a discussion of the parameters, and the LabTalk function syntax for each multi-parameter function.