Originally Posted by
eddibabyyeah
The system doesn't know who has a visa. It knows the passport which the guest is travelling on & knows that a visa is required for the destination. It then stops online checkin, because a manual check of that visa is required by staff at the airport. If online checkin proceeded, the guest could get as far as the gate or possibly onboard the plane without a visa check.
That's not quite correct and depends on the destination country,
In the case of Australia, a message is sent to Australian DIBP using the APP/API system and the response would have come back that the OP didn't have a visa (e-Visitor assuming they're a UK passport holder) registered against the passport they check in with. This is a simple Ok/Not ok in the check-in record.