ProClass like NHS eclass is derived from NSV.
There is more divergence from NSV headings, and things have been simplified with the number of categories reduced from 2,500 to around 1,500. Proclass is generally used in local government and 7 out of the 8 regional centres of excellence use ProClass.
ProClass is maintained by Coding International.
e2class is a more flexible hierarchy than NSV, and so e2class for Local authorities can support ProClass headings linking down through 4th and 5th level NSV heading to the underlying codes.