This routine returns the PDB or mmCIF file name of each generated loop model, usually of the form 'foo.BL000X000Y.pdb' (where root_name (or sequence)=foo, id1=X, id2=Y, and file_ext=.pdb). You can redefine this routine if you don't like the standard naming scheme. For typical LoopModel usage, id1 is the loop model number, running from LoopModel.loop.starting_model to LoopModel.loop.ending_model, and id2 is the comparative model number, running from AutoModel.starting_model to AutoModel.ending_model.
See also AutoModel.get_model_filename().