This option is used to make a create / edit the old .mf mapfile option in the project.
Continue down the page to see the Create/Edit .mf Panel options.
To view the Create/Edit .mf Panel, Use: File => Mapfiles => Create/edit .mf
This panel has become outdated in 12d Model, it can still be used to create or edit the old .mf file types, however the new file type .mapfile is the one to use.
Here is a link to the 12d Wiki post for the .mapfile: .mapfile