SSEA Rating and SEPL Rating
They are both pilots licence ratings NOT catergories of aircraft. There isn't a SSEA aeroplane catergory.
An NPPL who holds a SSEA rating may only fly single engined piston propellor aeroplanes that do not have an AUW that exceeds 2000kg. May only carry three passengers. Cannot fly at night or in IMC nor fly outside of the UK.
BEagle has explained all on another thread. However it is as follows; The CAA have granted an exemption (currently for one year) for those who hold a JAA or lifetime UK PPL to fly using a Medical Declaration as part of their licence signed by their GP in place of the JAA Medical certificate. BUT they are also restricted, if doing so, to the same extent as the NPPL holder. The Medical Declaration is the same as that which the NPPL holder is currently required to have issued, if they do not hold a JAA medical (some may).