The single major difference between a category ‘B’ licence and category ‘C’ licence lies in the authority of releasing an aircraft to service. 

The holder of a category ‘C” licence may “issue a certificate of release to service [CRS] for base maintenance carried out on a large aircraft”, and only if that person is type rated on that aircraft. 

By contrast, a category ‘B’ licence holder may “issue a certificate of release [CRS] to service for an aircraft covered by the licence if the maintenance was not base maintenance carried out on a large aircraft”. (Reference: Part 66 MOS, Pages 6 to 8)

In summary, a category B licence holder can issue a CRS for a “small aircraft” while a category C licence holder can issue a CRS for a “large aircraft. (See the FAQ “What is the difference in definition between a large and small aircraft?”).

The larger “problem” with these licence privileges lies in the precise definition of “base maintenance”. 

See the FAQ What is ‘base maintenance’ with respect to aircraft engineering?