I could not find a guide of all tag field names that foobar2000 understands.

The more popular ones are easy to know, like
- Genre - [%genre%]
- Album - [%album%]
and so on.

However, tags like
- Subtitle
- Conductor
still elude me, and not knowing the correct strings I can't make them appear.

Where may a find a complete guide?

There's no list of all tag field names because you can define your own and foobar2k will still understand it. Subtitle = %subtitle% and conductor = %conductor% in the same sense that you can add a custom tag field "joy to the world" to a track and access it with %joy to the world%. The brackets around it in [%genre%] is just TAGZ syntax to display nothing if the field doesn't exist (instead of a question mark, for instance). The titleformat_help.html file in your foobar directory shows example usage. Also, this may be useful if you want to know the fields that fb2k 0.9 has remappings on.

Look at the titleformat_help.html file that Silverbolt mentioned, and know that you can right-click on a track and select "properties" to see the metadata fields and tech info fields available for that particular track.

Thanks, Silverbolt and qnqfqf!

I was aware of the document "Foobar2000:Titleformat_Reference", but it doesn't seem to be complete (at least for a novice).
Stating my problem another way:
In a given MP3 file, I had filled with MediaCenter the Comment tag as "Moscou Concert".
MediaCenter and Collectorz read it OK, but fb2k reads (I used %comment%) it as "Media Jukebox: Tool Name: Media Center, Media Jukebox: Tool Version: 11.0.309".
(By clicking the right button to read Properties, one can see that the comma stands for a carriage return).
In that same file a date, "1960", appears to both those programs, but again not to fb2k.

Questions:
To get "Moscou Concert", what variable name should I have used?
How to get the date? (Tried %date% and %year%).

Out of curiosity, what are those two lines of information fb2k's reading? (Neither MediaCenter nor Collectorz display those lines that begin with "Media Jukebox...").

Incidentally:
- MediaCenter11 shows "Moscow Concert" under Comment, while Collectorz shows it under Notes);
- MediaCenter11 shows "1960" under Date, while Collectorz shows it under Year.

Try re-tagging it with foobar with the information you want.

Many, many files...
And I would miss the opportunity to learn about what's happening.

Could you post a screenshot of what your Properties window looks like for one of these files?

Two screencaps:
- Cap1a shows fields as written and read by MediaCenter11; note the presence of the Comment field filled with "Comment" and of the Date field filled with "1960";
- Cap2 shows fields as read by fb2k; note that the Comment (%comment%) here comprises two lines with different information and that Date doesn't appear (neither as %date% nor as %year%) and also the Properties window.

ID3v2.4 fields Subtitle and Band/Orchestra (this one I named as Ensemble in fb2k), all them very important for proper Classical music cataloguing, appear correctly in all three programs.

ID3v1 fields seem to be read by any program.

fb2k and MC11 screen caps

Could you post the exact titleformatting for your Date column? And it looks like MediaCenter adds a few tags of its own that it simply hides it from the user. Try [$meta(comment,1)] maybe? [$meta(comment,0)] for fb2k 0.9

Great!
[$meta(comment,1)] reveals the actual content of the Comment field.
However, if the field has no content, those two lines beginning with "Media Jukebox" that have been added by MediaCenter still appear.
Is it possible to hide them from view?

In what regards Date I'm embarrassed to say that [%date%] is indeed working.
I'm sure (am I??) I had tried that (besides [%year%]) and got no results...

Many thanks - I have really been drawn by fb2k, but early difficulties were annoying me.

Try

which basically translates to 'if the comment doesn't start with Media Jukebox, show the first comment if one exists.'

You can also tell MC 10 & 11 not to put certain tags into the file.
It will still keep them in its library for use.
I disable writing the comment and rating fields because of all the extra info written to the files.
If you do this, make sure to keep a backup of the MC library because ratings will not be imported if you lose the library and import files.

I use Comments as an intrenal tag to add information that won't fit in other fields, as in the example below:

Genre: Classical
Album: Kremer plays Beethoven and Berg
Track: 1
Composer: Beethoven
Title: Concerto for violin op.61 in D
Subtitle: I. Allegro ma non troppo
Artist: Kremer
Ensemble: Academy of St. Martin on the Fields
Conductor: Marriner
Date: 1977
Comment: Cadenzas by Schnittke

The cadenzas played by Kremer in this performance appear as Comment; this is accessory information and shouldn't be added to Title or Subtitle in order not to interfere with proper sorting.
This is often useful in cataloguing classical music and possibly other genres.