How notes are spelled
Every scale and chord is stored as a degree formula, such as 1 b3 4 5 b7, and not as a list of steps. Steps cannot spell notes correctly: F# major has to contain E#, not F.
To spell a note, Chordfolk takes the letter that is the right number of steps above the root's letter, then adds the sharp or flat that makes the pitch come out right. The notes inside a scale are always spelled this way, so D major has F# and F major has Bb.
If a note would need more than a double sharp or double flat, Chordfolk does not respell it quietly. It tells you, and suggests an enharmonic root to try instead.
How CAGED shapes are placed
Each CAGED shape is anchored where the root falls in the matching open-chord shape. Take r as the fret of the root on the shape's anchor string. The C shape covers frets r-3 to r, with its anchor on string 5. The A shape covers r-1 to r+3 on string 5, the G shape r-3 to r+1 on string 6, the E shape r-1 to r+2 on string 6, and the D shape r-1 to r+3 on string 4.
The windows are defined on major keys, and every scale note inside a window belongs to that shape. Along the neck the five windows repeat in the order C, A, G, E, D.
Every 7-note scale uses the windows of a reference major key. The major and natural minor systems use the parent major key (A natural minor uses C major). Harmonic minor uses the relative major of its parent (A harmonic minor uses C major), and melodic minor uses the parallel major (A melodic minor uses A major).
In harmonic minor the 7th rises one fret from the reference key's note, and in melodic minor the 3rd drops one fret. A moved note stays on its string unless that puts it more than one fret outside the window, and then it moves to the neighboring string. The panel names the reference key when it differs from the root.
CAGED needs strings tuned in fourths with a major third between strings 3 and 2, so it is offered only for tunings with that pattern.
How boxes and three-note-per-string patterns are built
Boxes (for pentatonic and blues scales) and three-notes-per-string patterns (for 7-note scales and some others) are built the same way. Pattern k starts on scale note k on the lowest string, at the lowest fret where that note occurs.
From there Chordfolk takes the next two scale notes on that string (three for 3NPS), moves to the next string, continues with the next scale note at the fret where that pitch sits, and repeats up to the highest string. It uses absolute pitch from the tuning, so this works in any tuning.
Boxes and 3NPS positions are numbered by the scale note they start on, not by place on the neck, so box 5 can sit below box 1. Every pattern sits at the lowest spot on the neck where no fret is below 0.
How fingers are suggested
Suggested fingers are computed for every position and never stored. Open strings get finger 0. The home window is the four frets starting at the pattern's lowest fretted note, with one finger per fret from index (1) to pinky (4).
For each string, Chordfolk uses the four-fret window closest to home that holds all of that string's notes. A string that needs a different window is marked as a shift. If a string's notes span five frets, it uses fingers 1, 2, and 4, and marks the pinky note as a stretch.
Shifts and stretches are drawn with a dashed outline, so you can see where the hand moves.