The cylinder is normally a twin coil vented type (for use where the hot water has its cold feed from a header tank in the loft), top coil for the boiler, and the lower coil for the solar input.
A pressurised (unvented) twin coil cylinder can be utilised where no header tanks are installed, and instead mains pressure is fed into the cylinder which gives greater flow to the taps i.e. no pressure problems- such as showers in a loft conversion. (Note- unvented systems can only be installed by a competent trained installer)
A thermal tank (store) can also be used, mains water pressure is supplied to the tank to enable high pressure to the outlet taps, but the heating system is vented with a header tank. This avoids the extra costs of running the pipework/controls of the unvented cylinders, and many alternative multiple energy sources can be added, i.e. wood burning stove/air source heat pump etc. This also has the added benefit of solar contributing to the space heating as well as the hot water.