This record works together with the control_mesh_generate_truss, control_mesh_generate_beam and control_mesh_generate_trussbeam records.
With macro_0 etc. you can specify the indices of control_mesh_macro_* records. Then the trusses (or beams or trussbeams) will only be generated for nodes coming from the mesh generated by the macro records with the specified indices.
This is handy in case you generate two neighbouring meshes with macro's, and want to generate the elements (trusses or beams or trussbeams) in between these two meshes. Normally, both the meshes would get the extra truss (or ..) in case you use a geometry_line or so to specify that the new elements should be generated between the two meshes (this is so, since the nodes of both meshes are located on the geometry_line). With the present control_mesh_generate_truss_beam_macro record however you can specify that the new elements should only be generated by looking at the nodes of some of the meshes, and so no double new elements will be generated in between the two meshes.