A system boiler is a boiler that is part of a package of closed loop ie pressurised heating system with an external vented or unvented hot water cylinder. It usually has an integrated pump, integral expansion vessel(additional vessel may be required dependent on system size), auto-bypass (most models).
There is also a variation called a 4 pipe system boiler that includes an integral divertor valve. For this setup no external divertor valves are necessary as long as there is a flow and return pipe directly to the hot water cylinder and you are able to attach an NTC sensor (in some setups you can work using the cylinder stat although it will not be possible to read the temperature.
System boilers with the right controls may support Priority Domestic Hot Water setup. Priority Domestic Hot Water (PDHW) . This is where the hot water is operated at fixed temperature and the heating at variable temparature dependent on room sensor or weather compensation