[sciurius]

ChordPro tries to recognize what is between the brackets as something that looks like a chord. In its default setting, that should be one of the letters C, D, E, F, G, A, or B, optionally followed by sharp or flat, optionally followed by things like m, maj, 7, aug, etc.

The chord may be parenthesised, e.g. [(A)]. This is treated as an A chord, transposed, and shown with parens. Except for NC (and N.C.) and spaces, eveything else is flagged as 'unknown chord' and treated as an annotation. So [+D] is treated as [*+D].

It is possible to define non-conformant chords but these cannot be transposed:

{defined: +D base-fret  1    frets  X  X  0  2  3  2  }

See also: https://www.chordpro.org/chordpro/chordpro-chords/