Practical salinity scale 1978 (PSS-78) is used for ionic content of seawater determined by electrical conductivity. Salinities measured using PSS-78 do not have units, but are approximately scaled to parts-per-thousand for the valid range. The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS-78 measurement values. The addition of PSU as a unit after the value is "formally incorrect and strongly discouraged".
Practical Salinity Unit