Using the FIS (see here for how to load FIS to GCD FIS Library), and associated surfaces loaded in previous step, you can run the FIS to derive an error surface. The video demonstrates this:
This video shows how you can calculate an FIS error raster using a pre-loaded FIS and associated surfaces as inputs:
For more information on fuzzy inference systems and how they can be used to calculate elevation uncertainty surfaces, see here. For conceptual background on fuzzy inference systems in general click here.