You can create multiple bookkeepings in System One, for instance, to send invoices on behalf of your artists' legal entity.
A bookkeeping refers to a company, that represents the legal entity that owns the bookkeeping.
Before you can create a new bookkeeping, you should make sure this company exists in your addressbook.
Every document in System One has a unique document number, for instance SAA-N22000012.
The first 3 letters/numbers of a document number are the bookkeeping code, in this case SAA.
Every bookkeeping has to have a unique code.