Do I need a negative or positive shower pump?
Positive head shower pumps are for gravity-fed systems only. As a rule, if your cold-water tank is above the shower (maybe you have a shower in a converted room downstairs) and you have a flow rate as little as 0.5 bar, then you will need a positive head shower pump.
However, universal head shower pumps, also known as negative head shower pumps, do not rely on gravity like their positive counterpart, and are used ideally when the shower is positioned above the cold water tank in your home. With that being said, if you are in about what pump is suitable for you, a universal shower pump is the best option.