Any aircraft that is thrust forward by a propeller powered by one, piston driven, engine is
by definition an SEP.
However not all SEPs fall in to the EASA 'SEP Class' - The 'TMG Class' for example.
Given the context of this Thread I am sure that when whopity said "SEP" he was simply abbreviating 'SEP Class'
I wonder what Class an aircraft driven by a Wankel engine falls in to?
I'm not sure about EASA aircraft, but AN-2 is a SEP that needs 2 pilots
If it needs two pilots it does not fall within the 'SEP Class'.
The 'SEP Class' means Non-High Performance, Single Pilot, SEPs. Any other SEP (which is not a TMG) would need a Type (or possibly an HP Class) Rating that would only be valid for one year and could not be revalidated by experience.
Revalidating an SEP Class Rating by experience requires that experience to have been obtained in an 'SEP Class' - with certain possible exceptions (eg valid experience gained in a TMG) which are stated in the regulations.
Since it is a Single Pilot Rating I cannot see that any Multi Pilot flying experience could be counted towards the revalidation requirements of an SEP Rating.