Systen boiler or heating only boiler

[Alfies Revenge]

I was looking for a boiler to replace and old one. The system will still have a hw cylinder. In a brochure i have there are system boilers and heating only boilers, whats the difference

 

[EliteHeat]

system boiler is sealed system and has pump expansion tank etc, heat only has nothing and relies on all existing gear, pump, F&E etc