What does ppm mean in practice?
For aqueous solutions, ppm is sometimes used as an approximation of mg/l, but the exact conversion depends on the density and how the result is defined.
When does it matter?
On a TDS meter, ppm is a value calculated from EC, not a direct measurement of the weight of all ingredients.
Which solutions are related to it?
Always provide the measured parameter, method and unit, instead of treating ppm as an independent water quality characteristic.