How it reads your text: a line counts as a chord line if every token on it looks like a chord (root A–G, optional flat/sharp, optional 7/9/maj7/sus4/dim/m/etc, optional slash bass). If a chord line is followed by a lyric line, the chords get inserted into the lyric at the matching column. If a chord line stands alone (an instrumental bar), it’s output as a line of bracketed chords. A whole line wrapped in single brackets, like [Verse 1] or [low C rumble], becomes a {comment: ...} line, unless the bracket only contains a single chord (like [C]) or a time signature (like [3/4]), which are left as ChordPro recognises them directly. Lines it isn’t sure about are passed through unchanged, so it’s worth a quick read over the output before you use it.