Check this box only if you have previously created the required reference curve between the two surfaces.
A variable radius fillet surface has different radius values at different locations along its length.
To create a variable radius fillet, first create a reference curve that
passes along the area to be filleted. Arcs with user defined radii will
be located along the reference curve at user defined locations. These
arcs are used to create the fillet. The reference curve should be created
as a cutter intersect spline between the two surfaces. To create the reference
curve, see Cutter Intersect 
on the Create > Spline
menu. The Tool total radius value on the Cutter
Intersect Tool Information dialog box should be set equal to the
largest radius used in creating the fillet.
A partial variable radius fillet surface can be created by trimming the reference curve.