When people refer to windows as being PVC, PVCu or UPVC, they’re all referring to the same thing: Unplasticised Polyvinyl Chloride. The confusion comes from naming conventions and contractions.
In the 1980s, the UPVC acronym was switched to PVCU to ensure consistency with other countries around the world. This was done with the best of intentions as in many languages, the noun (in this case, Polyvinyl Chloride) comes before the adjective (Unplasticised).
But due to the material’s prominence and popularity, many kept referring to it as UPVC. PVC comes from people dropping the Unplasticised part completely.
When it comes to windows and doors, no matter if it’s PVC, PVCu, UPVC or even PVC-U, they’re all the same.