SEP = Single Engine Piston (JAR-FCL term)
SSEA = Simple Single Engined Aeroplane (NPPL term)
SLMG = Self Launching Motor Glider (NPPL term)
There is NO SUCH THING as a 'Group A' aeroplane any more - and hasn't been for over 5 years. Unfortunately some unprofessional organisations still persist in using the term though.