Nath Jones explained it well.
My tap water is around 300ppm and my static tank is 1000 litres (1 million millilitres) - so an unfiltered tank must contain 300ml of dissolved solids.
For the sake of math, lets say my system isn't running as it should, and is pumping water out at 1ppm,.. only removing 299ml of dissolved solids,... what would it take to get my water from 1ppm down to 1ppb???
At the very most,.. if the parts per billion system worked perfectly and removed every trace from the water, the best it could do is remove the final 1ml of dissolved solids,.. meaning that its best possible performance produces water only 0.333% purer than a standard system.
PPB is marketing bull - plain & simple. If you think it works better for you, you're simply walking proof of the placebo effect.
Same equation but explained slightly differently
1000 litres of water at 300ppm is an easy calculation. 1000 litres is actually 1million millilitres/grams, so to reduce this to 1 ppm requires the removal of 299 grams of dissolved solids. Simples.
So - I now have 999.701 litres of water reading 001ppm (or 1000 parts per billion) so divide 999.701 by a billion parts & multiply the result by 1000 parts of impurities and you get a result of 0.000999701 litres or 0.999701 grams
So my ppm system removes 299 grams of dissolved solids - the Ionics ppb system removes just under an additional 1gram.