Callithumpian, thanks for the clear and concise info.
So basically the three options under "File Information" are for when you use an outside program to edit tags, right? As far as I can tell there is no way to change tag information with MJ without also changing the database; so if you always use MJ, then you never need to use these three fields.
""When doing an "Update Library":-
1) "Do not update database fields" - self explanatory, I guess. Sometimes you want to eliminate DataBase entries for which there are no files - like when you've moved some files around using explorer or some such."
See, I thought that "Remove files which do not exist" did this. And if I am right, then I can't see why anyone would want to 'update' their library by selecting 'do not update'... Unless, of course, this option works in conjuction with the 2 "clean-up" options. But that doesn't seem to work either, because if I select "remove files that do not exist", surely I can't also check "do not update database fields"?