Where the reference and tolerance live
Not here. A4 and the tolerances belong to the instrument, because they are properties of the accordion and not of the app: a concert box is held tighter than a folk one, and a 1938 instrument is not at 442 because you would like it to be. Set them when you describe the accordion, and every session on it starts from there.
What is here is the default the tuner uses when there is no session - when you are just checking a note.
Audio
Input device. The microphone the tuner listens on. Switching it restarts a running tuner.
Mains hum filter. Notch out hum the microphone picks up from the power supply: 50 Hz in Europe, 60 Hz in North America. Worth turning on if a reading wanders by about a hertz with nothing playing.
Clock correction. Corrects a sound card whose sample clock runs fast or slow, in parts per million. Leave it at zero unless you have measured the offset - this is not a wizard and it will not find the number for you. Modern converters are within a few ppm, so on most hardware it changes nothing.
This is the only setting about your computer's audio hardware. The thing the app calls Calibration sweeps the instrument, and is unrelated.
Reading
Deviation unit. Cents or hertz. A reed is filed in cents and a beat is heard as a rate; every number arrives from the tuner in both, so this costs nothing to change.
Reeds per note. How many reeds the tuner tries to resolve in one note. A pulled register overrides it - the register is the reed count.
Note naming. International (C D E F G A B), German (C D E F G A H), Polish, or solfege.
German naming writes H for B natural and B for B flat - a technician reading "B" for the wrong pitch tunes the wrong reed. Polish naming also writes H for B natural, but it spells the black key sharp rather than flat: C Cis D Dis E F Fis G Gis A Ais H. There is no B in it at all, which is how the note is reached when a reed is filed upward.
The names change everywhere at once, including in the printed report.
You drive to zero. The same measurement, said two ways: pick The goal and read Curr, driving it to the goal, or pick Zero and read Error, driving it to zero. Pick the one you have in your hands.
Behaviour
Hold the reading once it locks. The figure freezes the moment the tuner is sure of the note and stays frozen until you move on. For filing: your hand comes off the keyboard and the number is still there.
Keep updating on a pinned note. With a note pinned by hand, keep the numbers moving even when the tuner is not certain - for watching a reed come into tune as you file it. Off, only a locked reading repaints, so a reed that is still settling does not dance.
Both hold the reading still. The engine goes on measuring behind them, and a pass being recorded is unaffected.
Appearance
Theme (system, light or dark), language, and whether closing the window keeps the app running in the tray.
Note sounds are not here - they are a key on the tuner's own toolbar, next to the note controls. They were in Settings once and it was the wrong place: the only control in sight was the volume, which does nothing for a sound that is switched off, so turning it up looked broken.
Your workshop
The letterhead your reports print under: name, address, website and logo. It is off by default on every report; this is only what goes in it when you ask for it.
Settings is a room, not an overlay
Walking into it stops the tuner, as walking into any room other than Tune does. That is deliberate: the engine holds the microphone open and runs a DSP pipeline twelve times a second, and none of that is doing anything for someone filling in a form. It costs one keypress to come back and it costs nothing else - an open pass stays open and recording stays armed.