iTunes saves most of its track oriented metadata within the files...but unfortunately not all. The import/export process with iTunes is about exchanging the values which iTunes does not store in the files but only in its database. Yate supports seven of these database only fields and stores them in the file with the thought that it's where they belong. When you import, if some of the values you're bringing in differ from the values in Yate, the files will be marked as modified. Storing them in the file also means that if you have to rebuild your iTunes database you have the information stored somewhere. Keeping the play count up to date in the files is a pain but not losing the ratings, start and stop times, skip shuffle, etc. would be more problematic.

Whenever you save a file that is linked to iTunes, Yate refreshes iTunes with all of the stuff that iTunes does store in the file. By this operation the fields in iTunes will stay in sync with the fields in Yate. This does not automatically update the 'special' values...only Export will do it.

You can see the fields that are imported/exported by selecting a bunch of files, linking to iTunes and selecting the File>Compare iTunes Values menu item. This display will show you all of the 'special' values and how they differ. You can also import and export from this display.

Some of this information is retained in custom fields and some in standard fields. Of course what is standard in one audio file type may be custom in another.

There is a full (hopefully up to date) field mapping table accessible from the main Yate help page under References (towards the bottom of the page).

If other taggers can get at custom fields, they can get at the data.