Selectivity and permeability are key properties to characterise a membrane. The selectivity indicates how well a membrane can separate. Comparing the composition of a mixture before the membrane and after it has passed the membrane gives an impression of the selectivity of the membrane for that mixture. If one component of the mixture does pass the membrane entirely and the other(s) do not, then the separation is optimal and you have obtained infinite selectivity.

The other key characteristic, the permeability, is a measure of the rate at which a component to be separated is transported through the membrane, for example in m³ of liquid per hour per m² membrane surface per bar pressure difference. Suppose you want to have 1000 liters of purified water per hour, and you have a pump available that can apply 3 bar pressure difference over the membrane, then you can use the permeability to calculate how many m² of membrane surface area you will need.

The ideal membrane has a high selectivity and a high permeability. In practice, a membrane structure is often a compromise: pores with small diameters can achieve good separation (high selectivity), but also have a high transport resistance and therefore low permeability.