Full definition
Single source of truth is defined as: A single source of truth is one authoritative place where the current version of every supplier document lives, so different teams cannot end up sharing different copies.
A single source of truth means that the certificate stored in procurement, the one attached in an email thread and the one on the client portal are all the same file, with the same expiry date. When they are not, every downstream decision carries hidden risk.
In practice this is usually a shared workspace with clear ownership per document, versioning and a share mechanism that links back to the workspace rather than distributing detached PDFs.