Diatonic size: 1 = unison, 2 = second, ... 8 = octave.
Quality label: 'P', 'M', 'm', or repeated 'A'/'d'.
The signed semitone span the name covers when its letters ascend. Nearly every name spans upward, but a second narrowed far enough crosses zero while its letters still rise — C# up to Dbb is a doubly diminished second spanning -1 — and the sign is what keeps that name reading as the same interval a measurement of those two notes reports.
The semitone span of the interval named by a diatonic number and a quality.
The inverse of the quality derivation in spelledInterval: it turns a name back into a distance, which is what transposing by a named interval or parsing one from text needs.