The previously registered condition applies to a person who was previously registered to vote in the UK, either before they left the UK or as an overseas elector and the previously resident condition applies to a person who was previously resident in the UK (including those who left the UK before they were old enough to register to vote).
The guidance covers the actions you should take as ERO to carry out both stages of processing these applications. It includes guidance on how to carry out checks of further information to be able to be satisfied that an elector meets the eligibility conditions for overseas electors and passes the identity verification checks, through use of the IER digital service, local data matching, and where necessary the exceptions and attestation processes.
It also covers absent voting arrangements for overseas electors and how overseas electors will be shown on the register of electors.