A unique pupil number (UPN) identifies each pupil attending a state funded school in England. Allocated on first entry to a school, the UPN is generated using the nationally specified formula and is expected to remain with a pupil throughout their school career, regardless of any change in school or local authority. You can read the DfE guide to UPNs here: DfE Guide: Unique pupil numbers
Temporary UPNs are allocated when a school receives a pupil who may already have a UPN, but the receiving school does not yet know the UPN (for example, a CTF from the pupil’s previous school has yet to be obtained).
The formula for temporary UPNs is identical to that for permanent UPNs, except that they will have a letter at the end, for example; X00180001701A.
Temporary UPNs should be replaced immediately once the valid UPN is known with the temporary UPN being reported in the ‘former UPN’ field.