id3 tags include a media tag TMED which would be appropriate, but is unsupported in Rockbox.
Same with TXXX, user text frame.
A vorbis comment tag field could also be added, MEDIA=, but again unsupported.
In metadata_common.c I found these lines:
else if (strcasecmp(name, "comment") == 0)
{
p = &(id3->comment);
}
I believe I could add the following lines in order to have Rockbox parse the ogg vorbis comment field labeled DECSCRIPTION by Easytag and have access to it using %iC:
else if (strcasecmp(name, "description") == 0)
{
p = &(id3->comment);
}
I decided to take another look at the disc field.
Using Easytag to change the field in both mp3 and ogg files, I was able to enter various letter and number and other characters into the field. I was confused by the fact that Easytag doesn't display the tag correctly, but the characters are indeed in the field.
Rockbox doesn't allow # and % to be tested in the field, but does allow them in the field.
Otherwise, ! $ & work to be tested and letters, of course.
So I was finally able to output what I wanted using the following line. Output depends on whether the disk field is empty, has a number, or starts with L.
%ac%?in<TK %in %?if(%ss(0,1,%ik), =, L)<LP %ss(1,-,%ik)|%?ik<CD %ik>>|%?ig<%ig|%d(1)>>%ar%fb kb/s
So all is good and I don't need to use the comment field.