What a session holds
A session is one job on one accordion: the instrument it is on, its beating curve, every pass you have recorded, and the reference those passes were measured against. It saves to a file. There is no database and nothing to sign into.
It also takes its own copy of the instrument at the moment it is created. That is what lets a session from a year ago still open, still print, and still mean what it meant, even if you have since re-described the accordion or thrown the description away.
You do not need one
Open the tuner and play. Nothing is created, nothing is saved, and nothing is asked. The app calls this Just tune: check a note, leave, and no trace of it exists. A session is something you opt into when you are doing a job worth keeping, never a toll gate in front of a tuner.
Passes
A pass is one trip through the instrument, caught in one sitting.
Arm recording and play. Every note the tuner locks onto is written to the open pass, stamped with the register you pulled and the bellows direction you set. Enter records the note the tuner is on even if it has not locked; Backspace takes the last one back.
A pass freezes its reference when it starts. Every reading in it is measured from that A4, and you cannot move the reference while it is open. This is not fussiness: a pass measured against a sliding reference is a set of numbers you cannot file by, and it would look exactly like a good one.
The same note can be recorded more than once. Each is a take, the card keeps them all, and the last one is the one shown.
Recording survives leaving the room
Walking out of the Tune room stops the engine, but it does not end the pass or disarm recording. Go and read the table, come back, press Start, and you are still filling the same pass with nothing lost.
The three views
A session answers three questions, and there is a view for each.
Recorded tuning
What you measured. Every pass is a tab across the top; pick one and you get its graph and its table. Every note it caught, in order, with each reed under the rank it came out of. One table per pass, because a pass is one sitting and mixing two would mix two afternoons of an accordion's life with somebody's file in between.
Cells in an open pass can be corrected by hand. The backend recomputes the row; the number you type is read in whatever unit the table is showing.
Desired beatings
The beating curve: what the tremolo should be, note by note. See beating curves.
Before and after
Two passes side by side. Pick which two - the defaults are the first and the last, which is what "did this morning help" usually means. Every reed gets a verdict: now in tolerance, fell out of tolerance, closer, further out, unchanged.
It prints no number it was not given. The difference between two errors would be a number the app invented between them, so it does not do that; what it derives is the verdict, and that rests on the backend's own tolerance flag and on comparing two of its magnitudes.
If the two passes were recorded against different references, it says so. The numbers are still what was measured, but the two are not directly comparable and it will not pretend otherwise.
Housekeeping
Sessions are plain files. Back them up the way you back up anything else. Import and export move them between machines, or to somebody else.
Deleting a session takes everything recorded through that accordion with it: every pass, every reading, and the curve. It cannot be recovered from audio, and it cannot be undone.