Glossary · Performance and reporting
The book of record is the authoritative set of positions, transactions and balances from which every downstream number — performance, billing, reporting — is calculated.
A firm with two books has two answers to every question and no way to say which is correct. The practical test is to ask, for any figure a client sees, which single system computed it.
Numbers that are synced between systems were computed twice. Numbers that are read from one place were computed once.