The recovery key — what it is and where to keep it
Your books are encrypted from the first bill; copying the file gets nobody anything. There is nothing to switch on and nothing to remember day to day. The one thing it asks of you is the key you were shown at setup.
Where to keep it
At home, in a drawer — not in the shop, not stuck to the monitor. A key beside the computer protects nothing from the person who takes both.
Where to find it again
While the machine still works you can look it up under Settings › Encryption. The day it stops, that piece of paper is the only thing that opens your books on a new machine. Nobody can recover it for you. Not us, not anybody.
If the computer dies
Install InBilling on the new machine, restore your most recent backup, and give it the recovery key. That is what the backup and the key are for.