Pseudonymisation

Pseudonymisation replaces the obvious identifiers with a reference: customer 4417 rather than a name.

It is a genuinely useful reduction in exposure and it is not anonymisation, because somewhere there is a key that maps the reference back to the person. Under UK GDPR the data remains personal data, with all the obligations that brings. The distinction gets blurred frequently, usually in the direction people find convenient, and the consequence is teams believing a dataset has left the regime when it has not.