Users don't care about tagging that much but as a developer of a tagging tool I have to care. :) So here's what I'm dealing with right now:

In the newer tagging formats the fields are arranged in an expandable list. There are standard fields to use (for MP3 id3v2 and OGG comments) but there is no restriction on how many times a field can be used, like comments. Also the ordering of the fields only matters between fields of the same name.

For example, iTunes uses the id3v2.2 comment field (COM) three times. The user's comment is the last one. The other two are the file checksum and a CDDB lookup number used by iTunes (when you rip a file from CD). When I'm reading in these file formats I'm only concerned about a few of the fields, like that last comment.

When a user edits a file in AudioMan, even though I never use the other two comment fields I don't want to erase or modify them. So on a file write I will have to keep track of the fields I don't modify and rewrite them back to the file in the same order. Then I won't screw up fields that AudioMan isn't concerned about (yet) and that other applications use.