The classification system started on the Midland, where it was very strictly imposed. It was modified under LMS and BR authority. It was also used practically on the LMS and LMR to assess loading criteria; the Crabs of both types were uprated - on paper - because the work they were doing on weekend excursions was in the 6P category and there weren't enough genuine Class 6 locos otherwise available to cover it. While this was a misuse of the system, it does show that it was in operation. While other Regions might or might not have used the ex-LMS system as such, I'd suggest that there was some similar means of classifying locos and loadings in force.