Full definition
Chain of custody is defined as: Chain of custody is the documented trail of who uploaded, reviewed, approved and shared a supplier document, from the moment it entered the system to every place it has since been sent.
Chain of custody answers the question a regulator or auditor eventually asks: how do you know this is the real certificate, and who has seen it. A clean chain shows the original upload, the reviewer, any approvals and every share event with a timestamp.
Storing documents in shared drives without an audit log breaks the chain. Rebuilding it after the fact - from email threads and memory - is where most compliance findings originate.