Certain substances which meet the criteria of (i) preparation in highly pure state reproducibly and (ii) stability of solution over a reasonable period of time are designated as operational standards in aqueous solution of specified concentration. Their number is in principle unlimited but values are available now for 15 solutions. The values of operational pH standard, i.e. pH OS, are assigned by comparison with the reference value pH standard, i.e. pH RVS, in cells with liquid junction, the operational cells, where the liquid junctions are formed within vertical 1 mm tubes.
Orange Book, 2nd ed., p. 21 (http://media.iupac.org/publications/analytical_compendium/)