For example Cessna SET is a different class from Pilatus SET and so, you need 15 hours on either class before you can instruct on them. However, the Cessna SET class has more than one model in it - C206, 207 and 208 so you need differences training before you can instruct on one of the other models within the class, but not the 15 hours which only applies to different types or classes.

Cessna SET and Pilatus SET are both within „the“ SET class rating and are considered as variants (see rating list explanation).
915 b 3 i says, 15 hours pilot time within „the“ class rating.
where is the legal basis to consider both SET as different class ratings, rather then being within „the“ class rating SET?
they are listed individually for Admin, yes, and afford differences training as a common SEP(l) does

If you have an SET class in Section XII of your licence, you will see that it is entered as 'Cessna SET' or 'Pilatus SET', not just 'SET'. Each is therefore considered to be a separate class and so differences training is not sufficient - you need to undertake an appropriate class rating course at an ATO and pass a skills test on it before you can captain a new SET class. It therefore follows that you need 15 hours on the new class before you can instruct on it.

Thnx Stephen, but i still read it a bit different. I asked EASA to clear this one up, but unfortunately i got no responses on either of my 2 ways to ask for clarification.