Ah! Now I understand!
The training is at the discretion of the CRI in order for the candidate to be able to pass a skill test. For some ratings there are a number of elements that need to be covered which are part of the class rating. So for example someone going from a ML to a SSEA would need to meet the requirements of the SSEA, stall and spin awareness training as an example.
It is up to the CRI/FI conducting the training to ensure he requirements have been met. In the case of the NPPL it is all quite well load out on the NPPL website. For a JAA licence it's a case of looking at the requirements for each class rating.
Nothing to difficult.