Letters correspond to the ABC header lines that were found.
A double quote ('"') means that the tune contains accompaniment chords (such as "Bb" or "Em7").
For example, CNWZ would mean that the C (Composer), N (Notes), W (Words) and Z (transcription notes) header lines are included. Similarly, "OP means that there are chords, an Origin line, and that the tune labels its Parts.
XTK are omitted, since they are normally found in (almost) all tunes.