What it calibrates
The instrument, not the computer. Calibration learns the accordion's keyboard and sweeps its reeds, so that what comes out is a filled card rather than a setting.
It is not sound-card calibration. If your sound card's clock runs fast, that is the Clock correction field in Settings, and it is a different thing entirely.
A session has to be open: calibration measures onto the accordion a session is recorded on, and the readings have to land somewhere.
Why use it instead of just recording
Nothing. Free-form recording does the same job: arm it, play, and every locked note is caught.
The sweep is for when you would rather be told what to play than keep track of where you are. It walks the range note by note, in both bellows directions, on one register, and tells you the next one. On a full-size box that is several hundred readings, and losing your place in the middle of it is easy and expensive.
The readings it captures are the ordinary recording. Same table, same card, same report. The sweep only pins the note, sets the register and sets the direction for you.
Step one: the keyboard
Play the lowest key, capture it. Play the highest key, capture it. That is the range.
You can type it instead, on the instrument itself, if you know it. If the highest comes out below the lowest, the app says so rather than accepting a keyboard that runs backwards.
Step two: the reeds
Pick the register you are calibrating. One register at a time - an accordion is calibrated one switch at a time, because a register is what decides which ranks are sounding and therefore which columns the readings land in.
Then work down the list. For each note the sweep asks for Press and Draw: they are different tongues, they drift apart on their own, and they both get tuned. When the reading lands, the sweep moves to the next one on its own. Skip anything that will not sound; stop whenever you like.
If nobody has described the accordion's registers, the sweep still runs - its readings just land in numbered columns instead of named ones. That says less and claims nothing, which is the right trade: a reading filed under the wrong rank looks exactly like one filed under the right one.
The microphone stays live
Calibration is guided tuning: the microphone is open and the accordion is being played the whole time. It is the one room besides Tune where the engine keeps running, and the only exception to "leave the tuning room, stop the tuner".
When it is done
It tells you how many readings landed, and offers you the card. From there it is an ordinary pass: read the errors, open the box, file, and sweep again to check.