Selective Subdivision

Basically same thing as how you subidivide something in edit mode (Select -> Subdivide) but expanded to the modifier and node.

Subdivision surface modifier gets applied to specific edges/faces based on vertex group. (Edges with both vertex weights > 0 get subdivided).

Subdivision surface geometry node gets "selection" input.

Abrupt transitions in topology are handled using n-gons, or some kind of tesselation algorithm.