Here is the tags treated by metaaac:
%atom = (
# container atoms
ALB => { CONTAINER => 1, TAG => ALBUM, FORMAT => TEXT },
ART => { CONTAINER => 1, TAG => ARTIST, FORMAT => TEXT },
CLIP => { CONTAINER => 1},
CMT => { CONTAINER => 1, TAG => COMMENT, FORMAT => TEXT },
COVR => { CONTAINER => 1, },
CPIL => { CONTAINER => 1, TAG => COMPILATION, FORMAT => INTEGER },
DAY => { CONTAINER => 1, TAG => DATE, FORMAT => YEAR },
DINF => { CONTAINER => 1},
DISK => { CONTAINER => 1, TAG => DISCNUMBER, FORMAT => INTEGERS },
DRMS => { CONTAINER => 1},
EDTS => { CONTAINER => 1},
GRP => { CONTAINER => 1, TAG => GROUPING, FORMAT => TEXT },
ILST => { CONTAINER => 1},
LYR => { CONTAINER => 1, TAG => LYRICS, FORMAT => TEXT },
MATT => { CONTAINER => 1},
MDIA => { CONTAINER => 1},
MINF => { CONTAINER => 1},
MOOV => { CONTAINER => 1},
NAM => { CONTAINER => 1, TAG => TITLE, FORMAT => TEXT },
RTNG => { CONTAINER => 1, TAG => RATING, FORMAT => INTEGER },
SCHI => { CONTAINER => 1},
SINF => { CONTAINER => 1},
STBL => { CONTAINER => 1},
TMPO => { CONTAINER => 1, TAG => TEMPO, FORMAT => INTEGER },
TOO => { CONTAINER => 1, TAG => ENCODER, FORMAT => TEXT },
TRAK => { CONTAINER => 1, SKIP => 1},
TRKN => { CONTAINER => 1, TAG => TRACKNUMBER, FORMAT => INTEGERS },
UDTA => { CONTAINER => 1},
WRT => { CONTAINER => 1, TAG => COMPOSER, FORMAT => TEXT },
APID => { CONTAINER => 1, TAG => USER, FORMAT => TEXT},
PLID => { CONTAINER => 1},
AART => { CONTAINER => 1, TAG => ARTIST, FORMAT => TEXT},
GEID => { CONTAINER => 1},
AKID => { CONTAINER => 1},
ATID => { CONTAINER => 1},
CPRT => { CONTAINER => 1, TAG => COPYRIGHT, FORMAT => TEXT },
"----" => { CONTAINER => 1, TAG => ITUNES, FORMAT => ITUNES},
GEN => { CONTAINER => 1, TAG => GENRE, FORMAT => GENRE },
GNRE => { CONTAINER => 1, TAG => GENRE, FORMAT => GENRE },
As you can see a number of the tags are not currently mapped into a tag and therefore only appears when using the --verbose option of metaaac.