System of record
A system of record is the one system treated as the authoritative source for a given type of data, such as the CRM for accounts and opportunities.
In practice
When two tools both claim to hold the truth about a customer, integrations overwrite each other and teams argue about whose number is right. Naming a system of record for each object settles which value wins.
List each key object (account, contact, opportunity, subscription, product usage) with its system of record and the systems that only read from it. Set integrations so that only the system of record writes the master fields, and document how conflicts are resolved.