In the "Install Menu Set" script step, there is a checkbox option to "use as file default". Oddly, this does not actually change the default menu set. If you go to Manage Custom Menus, the "default menu set for this file" is unchanged. The only thing that seems to change is which menu set is installed when you run Install Menu Set / [File Default], which then installs the File Default previously set by the script step instead of the "default menu set for this file" specified in Manage.
I think it's pretty confusing to have a "file default" that is different from the "default menu set for this file". The terms are similar enough that it seems like they would mean the same thing.
1. "File Default" should be the name for the file's default menu set, which is active when the file is first opened, and which is set from Manage.
2. "Session Default" should be the name for the temporary, does-not-survive-close-and-reopen menu set, which is set by checking the box in the Install Menu Set script step.
3. "Default" should mean the "Session Default" if there is one, else the "File Default", and should be an option in the Install Menu Set menu.
I just think that'd be less confusing.