A source of truth is the agreed place to check a particular fact. It is an operational responsibility, not a guarantee that a system is always correct. The useful question is often “Which source governs this field?” rather than “Which system contains everything?”
Different sources may govern different facts. Your invoicing system may govern invoice numbers, while a product register governs product descriptions. Write those responsibilities down and make disagreements visible so someone can resolve them.
A small-business example
A retailer has three prices for the same product: a supplier's latest quotation, the buying team's costing sheet and the shop's current selling price. These are three distinct facts. Picking the newest number and calling it “the price” would destroy their meaning.
The team names each price clearly, assigns a maintained source to each and records when the value took effect. Staff can then investigate a discrepancy without overwriting a valid value from another context.
Try this
Choose one field that regularly causes arguments. Identify its intended meaning, who is responsible for correcting it and where an approved correction must happen. Document what downstream copies need updating. Check a real disagreement together and keep the resolution with the records.
Data dictionary makes the meaning explicit. Reconciliation helps find disagreements, while Version history records how an accepted fact changed and why.