What is in one
A page per item: brand, model, serial, where it is, when it went in, and what it cost if anyone knows. Under that, the service manual, the filters and belts by part number, how long the thing tends to last, what fails first and at what age, and what the warranty still covers.
Then a calendar. Not a generic one — the manufacturer's own intervals, adjusted for the climate the house is actually in. Freeze season in Minnesota is not pollen season in Georgia.
The buyer gets an account with all of it already in it. No setup, no typing, nothing to import.
Why it works at closing
The first six months of ownership is the highest-intent moment in this whole category. Somebody has just inherited twenty machines they do not understand, is already spending money on the house, and has no habit yet about how to keep track of any of it.
It is also the cheapest moment to be useful. A dossier that arrives with the keys gets kept. The same dossier offered in year four is a chore.
What the seller gets out of it
A listing that can answer questions instead of shrugging at them. The age of the roof, the model of the furnace, whether the water heater is still under warranty — in writing, before anyone asks. It is a small line item that makes the house look looked-after, because it was.