C dim 7 is, as its name infers, a type of seventh chord.
Seventh chords are 'four-note chords' consisting of a Root, a Third
of some type, a Fifth of some type and a Seventh of some type.
The notes of a Diminished Seventh chord are a Root, a flatted Third,
a flatted Fifth and a double-flatted Seventh.
The chart above lists an
'A' note, which is the
Sixth-tone
(with C serving as the Tonic: C=1, D=2, E=3, F=4, G=5, A=6 & B=7).
The name of the (double-flatted) Seventh is
bb7, or
Bbb .
In the group of triads in the chart, to be comprehensive, a
Diminished Chord would be included.
The notes of a Diminished chord (triad) are Root, flatted Third and flatted Fifth.
In reference to C as the Tonic, the note names would be C, Eb and Gb .
Also, a
Suspended chord (R, 4 & 5 or C, F & G) would be in the listed triads.