The head/pressure rule of thumb and why heated water needs expansion provision.
A useful plumbing rule of thumb: roughly 1 bar of pressure for every 10 metres of vertical head of water. More head above an outlet means more static pressure at it.
Water expands when heated, so closed hot-water systems need expansion provision (an expansion vessel or equivalent) to absorb the increase safely.
A pressure reference point
Standard atmospheric pressure is about 1 bar (101.325 kPa). A gauge referenced to atmosphere reads the difference from the local air pressure, which varies with altitude and weather. Absolute pressure includes atmospheric pressure: it is gauge pressure plus the local atmospheric pressure.
Gas cushions use absolute pressure
For a fixed amount of gas at constant temperature, halving its volume doubles its absolute pressure. For example, with atmospheric pressure approximated as 1 bar, a cushion initially at 1 bar gauge is at 2 bar absolute. Halving its volume gives 4 bar absolute, or 3 bar gauge. This idealised example explains the compressible cushion; use the system instructions for vessel size, pre-charge and filling settings.