What do you use custom tags, virtual tags, highlighting, and sending to external programs for? I'm just curious.
WELL...

Here's an example that shows all of the above (it doesn't show the external programs, but they were involved).
Custom Tags:
I use the Sort Tags for translation (as shown here) and grouping (for example, to collect Alison Krauss and Alison Krauss & Union Station under the same heading, and also the same folder on my drive).
Original Year is either the year the track was at its chart peak or (for classical) the year of composition.
Billboard Singles records the chart peak and number of weeks in the chart. The score is a number generated based on its week-by-week chart performance, weighted heavily towards the higher chart numbers (I have a spreadsheet for this, mostly not made by me, thank goodness!). If you're wondering why I bother with all this information... curiosity, the challenge of it, and it gives me an easy way to generate a quick playlist for a general audience. I have auto-playlists for each decade.
Track/Album awards are meant for Grammy's and Oscars, but I'll deal with them once I'm done with the Billboard stuff. Could be a while.
Location, hopefully self-explanatory: I wanted to see the origin of each artist or band (shown at right in the artist header). Also still in progress, especially since the Picard plugin that was tagging location broke thanks to last.fm.
Musicians/Personnel: this is data imported from Musicbrainz, mostly to be accessed when searching. The data on their end is far from complete, but it's better than nothing.
iTunes Media Type: used to mark audiobooks as audiobooks.
Compilation: same as the "iTunes Compilation" on the Settings tab (1 for yes, 0 or blank for no). I can't remember now how I was using it. Might get rid of it.
Owned: I'm currently in the process of acquiring physical copies of albums that I have a lot of tracks from. This tells me which CDs I already have (or in some cases LPs). It's also helpful because I own some compilation CDs, but have the tracks tagged with their original source, so I can see which compilation I got them from. I just got started with this project, so it's far from complete.
Virtual Tags:
That subheader in blue above the track name is a virtual tag called (wait for it) Subheader. This is what it looks like raw:
$If($And($IsMatch(<Billboard Singles>,.+)="F",$IsMatch(<Track Awards>,.+)="F"),$IsNull(<Grouping>,,$If($Contains(<Genre>,Christmas)="T",,<Grouping>)),$IsNull(<Billboard Singles>,,"[Billboard Hit "<Original Year>"] "<Billboard Singles>))$If($And($IsMatch(<Billboard Singles>,.+)="T",$IsMatch(<Track Awards>,.+)="T")," + ",)$IsNull(<Track Awards>,,[$First(<Track Awards>)" Grammy Awards] "$Replace(<Track Awards>,$Left(<Track Awards>,6),))
In plain English, it checks to see if either Billboard Singles or Track Awards has a value. If it finds either, it displays that (formatted variously depending on the circumstances). If not, it looks at Grouping (where I keep info like subtitles for multi-disc sets) and displays that, unless Christmas is in the genre tag (because I use Grouping differently for Christmas carols).
I currently have 11 virtual tags, although some of them aren't being used. I'll get rid of them if I need the space.
"Artist Group" is the one that processes the Sort Artist/Sort Album Artist values to create a consistent folder and heading structure. I made it a separate tag partly to simplify my already-complicated Auto-Organize formula.
"Date" looks at Original Year and fills in Year where it's blank, and is used for display in the playing track panel and also for my decade auto-playlists.
"Album Count" is that [#/##] figure you see to the right of the artwork, and shows how many tracks from an album are in my library.
"First Letter" is used in my auto-organize template and also sometimes the column browser.
"Artwork 2nd Line" is for Artwork view, to control what other tags are displayed with tracks based on their genre or genre category. For Musicals, it displays the performers for the track, for Classical it displays the composer but only if different from the artist/album artist, etc.
"Chart Score" takes that custom Billboard score and turns it into a raw number that I can sort by (also for the decade playlists).
Highlighting:
I use highlighting in a couple of ways. First, tracks which appear to be duplicates (based on file names that end in _1 or _2) are turned orange, and tracks which are not in MP3 format are turned red. Those most often turn up in the Inbox.
What you see at the right here are my "processing" icons. No icon means it's new and needs attention. The little tag means that I've processed that file with MusicBrainz Picard (where have a custom script to fill in the Publisher tag with "Tagged"). More than 90% are tagged right now.
Once I've confirmed that the track has acceptable artwork (at least 1000x1000 or best available - if MB can't find anything, I'll send it to Album Art Downloader), I change "Tagged" to "Clean Up", and the icon is a broom (this track is actually ready for that, I just haven't gotten around to changing it yet). The order is important, because Picard's biggest flaw IMO is that it will clear existing artwork, so there's no point until the initial tagging is done.
Once I go over the tags manually and am happy with everything, I'll change it to "Done" and it gets a big green check mark. (I only have about a dozen "Done" tracks so far, lol. That's long-term planning). There's also a big red alert icon that shows up if I put any other notes in the Publisher field, such as if I need to replace a bad track.
Custom Toolbar and External Programs:
See that line of icons at the top? From left to right those are:
*Send to Picard
*Rescan Files (used a lot less since Steven made rescanning instantaneous)
*Tag Editor
*Tag Inspector
*Send to Album Art Downloader
*Preferences
*Restart MB
I also have "Send to External Program" configured for:
*Google Images (Album)
*Discogs (Artist)
*Wikipedia (Artist)
*All Music (Composer)
...are you sorry you asked?
