The Consort meters that are able to measure Salinity (salt content of water) are based upon the conductivity measurement. It is expressed in a dimensionless value that represents the amount of grams of salt per kg of water (g/kg).
The Salinity measurement is based up on the Practical Salinity Scale of 1978 (PSS-78). The complete theory can be read on this website: https://salinometry.com/pss-78/.