The Fillet command requires that exactly two database curves or boundaries be selected, along with any combination of any entity types other than database curves or boundaries, for the command to be available. This command will create a filleting surface using three database curves or boundaries, two rails which must be part of the primary selection and a slope control curve picked within the command (a fillet surface is the 3D equivalent of the conic curve). Database boundaries (u-min, u-max, v-min and v-max curves) are also created and are considered child entities to the filleting surface.
Select the two database curves or boundaries that will serve as the surface rail curves, then Create, Fillet. Select the slope control curve. Adjust the Fit Tolerance if necessary to produce a surface. Raise or lower the ρ (rho) value as desired to pull the surface in closer to the slope control curve or to push it away, respectively. Click on OK to save the surface and close the panel.
The Fillet panel provides a text field for adjusting the ρ value as desired. Large values nearing 1.0 pull the surface toward the slope control curve. Smaller values near 0.0 push it away, effectively flattening it out. A Tolerance frame allows you to change the Fit Tolerance away from the default value of 0.001 when the set of curves it not producing a valid fillet surface. Uncheck Use Default to enter a custom value.