A source of truth (also known as a system of records) is just a dataset that has the ultimate authoritative say over data-correctness. When you create a copy of a data-set and you don’t treat it as the ‘source of truth’, one talks about a Derived data set.
Often it will be the first one to receive data and derived data will either copy-on-write or pull changes, like a data warehouses.
When there is a discrepancy between data systems, the source of truth will be the system with a canonical take or actual correctness.
Reference
Kleppmann, Martin, and Chris Riccomini. Designing Data-Intensive Applications: The Big Ideas behind Reliable, Scalable, and Maintainable Systems. Second Edition, O’Reilly Media, 2026.