This video shows how to install the macro, and how to run a simple demo.

The aim was to create a volume-based selection of faces or solids in FreeCAD. SelectionSet: define a “selection shape” (e.g. a box, sphere, or PartDesign body), then store which faces or solids of another object lie inside or intersect it as a SelectionSet in the model tree. SelectionSetLink: combine one or more SelectionSets (and/or direct shapes), add and subtract as needed, then apply the result to a FEM constraint, material, or any object with a References property—so you drive FEM geometry from your selections in one place.

https://github.com/HoWilgh/FreeCAD-macros/tree/master/FEM/SelectionSet