This blog is updated daily.
A general description is here.
A package with Fortran source code and perhaps C (but not C++) sources can request for its shared object/DLL to be linked by the Fortran compiler by including a line ‘USE_FC_TO_LINK=’ in ‘src/Makevars[.win]’.
The known reason for doing so is a package which uses Fortran (only) OpenMP on a platform where the Fortran OpenMP runtime is incompatible with the C one (e.g. ‘gfortran’ 9.x with ‘clang’).