These terms are not a common ...

These terms are not a common practice and therefore your question is puzzling. However I try to put an answer:

After the sequestratio of the ions, the desalinated fresh water contains some CO2 that normally gives an acid pH because its combination with water gives carbonic acid.  In thermal desalination the vented gases are rich of CO2 which is adsorbed by the distillate when condensing. In RO the free CO2 flows through the mebranes and eventually the peak concentration of bicarbonates near the membranes generates new free CO2 that flows into the permeate.

Hence the actual pH gives a measure of the quantity of dissolved CO2.

In normal water exposed to the atmosphere some CO2 (present in the air at eventually 400 ppm) is adsorbed up to saturation according to the Henry's law (natural presence of CO2 in all surface water). The comparison between the two concentrations (actual and natural saturation) may be of some interest in the evaluation of the quality of the desalinated water before any post-treatment which may modify the pH as desired for potable service.

It is not clear what pH could be considered the best for potable water. After examining the analysis of the best (and most expensive) bottled water I found that the majority has a slightly alkaline pH, but many have an acid pH.

so, what is the best ? I personally drink sparkling water which is artificially enriched with CO2 and I find it very pleasant.