Basic and Advanced Fillet

Basic: When this option is on, a simple arc fillet is applied between the two selected shapes. Fillet Radius: The value entered in this field determines the size of the arc of the fillet. Note that the size of the fillet should be appropriate (not too small and not too large) relative to the pair of faces that will be filleted, or otherwise the operation will fail.

Advanced: When this is selected the advanced fillet method is applied.
Circular, Elliptical, Bevel: These options determine the cross section of the fillet.
They are the same as the edge options found in the smooth plain rounding and smooth control rounding (see section 4.12.2). Note that, unlike the Basic fillet, the pick order and pick locations are significant for the Advanced fillet. For best results the faces should be selected using the On Edges option in the Pick Options dialog to identify the fillet faces. The first face selected is used as the orientation face for the Elliptical and Bevel cross sections. The first edge selected is used as the orientation edge for variable radius/distance fillets. For closed face edges, the click point is used as the start for variable radius/distance fillets.