Why describe an accordion at all
Nothing in the audio can tell you which rank a reed belongs to. A reed at 312.5 Hz is a reed at 312.5 Hz whether it is M1 or M2. The tuner hears pitches, not ranks.
So the instrument is asked instead. You describe the accordion once: its reed ranks, its switches, whether the bellows change the note. After that a register names the ranks it sounds, a take names the register you pulled, and every reading lands in the right column of the card without anyone guessing.
That matters because a reading filed under the wrong rank looks exactly like one filed under the right one. There is nothing to notice and nothing to catch it. You would file a real reed to a pitch nobody asked for.
You do not need one
The tuner works perfectly well without an instrument. If you have not described one, the tuning table numbers its reed columns instead of naming them. That is honest and it is fine. You can describe the accordion later, or never.
The registers
A register is what one switch turns on, and the app reads it straight off the switch's own symbol: the circle engraved on the accordion, divided into three bands - 4' on top, 8' in the middle, 16' at the bottom. Click the dots the way they are engraved, or pick a common register from the list.
What that switch sounds is read off the dots. The register is the reed count, which is why picking a register sets how many reeds the tuner listens for.
The rest of the description
Name. Whatever you call this box. Yours to name; nothing translates it.
Make, model, serial. Optional, and only for finding it again.
Reference pitch. This accordion's own A. 442 is common on newer instruments, 440 on plenty, and old boxes turn up lower. Every session on the instrument starts from here.
Reed and beat tolerance. How far a reed, or a beat, may sit out before the app calls it out of tune. This belongs to the instrument: a concert box is held tighter than a folk one.
The bellows change the note. Tick this for a diatonic button box, where a key sounds a different note pushing than pulling. Most accordions are not like this. Note that it does not decide whether bellows direction is recorded - direction is always worth recording, because every note has a press reed and a draw reed.
Keyboard range. Optional: the lowest and highest key. You can type it, or let the calibration wizard learn it by ear.
Photograph. For recognising the thing on your shelf.
Instruments and sessions
An instrument is a description. A session is a job done on one.
Describe the accordion once and every session on it inherits the ranks, the registers, the reference and the tolerances. Nothing is retyped.
When a session is recorded, it takes its own copy of the instrument. So deleting or editing an instrument later cannot rewrite history: an old session still opens, still prints, and still means exactly what it meant on the day. That copy is also what makes a session portable - send the file and the instrument goes with it.
Sharing
Instruments import and export as files of their own. If somebody describes a Hohner Morino once and does it properly, nobody else has to.