I think it has to be the solid line. Each surface must be tangent to the fillet.

3.1.9 Fillet Radius

An arc with radius of curvature respecting the limit(s) specified, connecting two fully defined surfaces tangentially with no inflection and perpendicular to the intersection between them. Unless otherwise specified, fillet radii may change in magnitude around the periphery of the boundary around which they are defined, but such changes must be continuous.

If there exists a discontinuity in tangency at the trailing edge of the intersection between the parts to be joined by the fillet, then the fillet surfaces may be extended behind the trailing edge to provide a closed aerodynamic fairing. This fairing need not consist of arcs of constant radius, but, relative to the fillet immediately preceding the trailing edge, it may be no larger in cross section than and no longer than three times the maximum fillet arc radius at this point.

That said it's specifying minimum convex and concave curvature not a fillet radius...

3.1.4 Concave and convex curvature

References made in this Article on curvature of aerodynamic surfaces refer to the part of the aerodynamic surface which is in contact with the external air stream.

When references are made to the curvature of a surface, without specifying an intersection with a particular plane, the local curvature at any point will be defined as the curvature of the intersection of the surface in question with a plane passing through a line normal to the surface at that point. The concave radius of curvature of the surface at that point will be defined as the minimum concave radius of curvature obtained when the intersecting plane is swept through 180 degrees around the normal line. The convex radius of curvature of that surface at that point will be defined as the minimum convex radius of curvature obtained when the intersecting plane is swept through 180 degrees around the normal line.

As an example, and for the sake of clarity, the aerodynamic surface of a solid sphere would be the surface where this sphere makes contact with the external airstream, and would be considered to be a convex surface.