the three ways salt in water are read through the electrode in your meter. ppm you know that. tds total dissolved solids, all measured through ec(electrical conductivity).
I use the ppm cause it easier to guage imop... furthermo, ppm are parts per million salts in your res> nutrients etc. and no tap usually has a ppm of 250 to 550 depending. (on luck mostly) ph also. but the ppm in a cup will register close to a 30 gallon tub full, a 5 gallon will read close to that of a 40 gal. the source deters the starting ppm's, then the nutes are added.. im drunk hope this makes sense..