Practical salinity from a conductivity reading, on the Practical Salinity Scale 1978 (UNESCO 1983). Salinity on this scale is a ratio of conductivities, so it has no unit — the "PSU" you see written after it is convention, not dimension.
What it assumes
Pressure is gauge, not absolute — a surface sample is 0 dbar, not ~10.
Temperature is in situ, on the ITS-68 scale the standard was written against.
The reference is standard seawater at S=35, T=15 °C, P=0, whose conductivity is 42.914 mS/cm.
Below 2 the scale is undefined; brackish and hypersaline water want a different one.