The degree of freedom mapping is generated when "

parseprob" is called by the "

mapdofbdr" function, and will generate arrays of dof maps for each dependent variable in the "fea.eqn.dofm" field.

https://www.featool.com/doc/feastruct.html#eqn_structEach dof map is an array of "n_local_dofs x n_grid_cells" and gives the global dof numbering on each cell (depending on the shape function for the dependent variable, but in the simple case of 1st order nodal basis functions is the same as the fea.grid.c grid cell mapping). The (row) order of the solution vector in "fea.sol.u" follows the dofm maps for each dependent variable in sequence.

So you would have to use this map and compute a reindexing from your partial mesh to your full mesh.