Whatman Type CF pH indicator strips have multiple segments of different dye-impregnated paper mounted on each plastic strip. The dyes are chemically bonded so they will not leach out into your solution. The use of several different segments gives an extremely clear visual comparison for every pH value.
The Type CS pH indicator strips have a printed colour comparison chart on each test strip and a central indicator segment which changes colour. This segment is then compared with the matching colour on the strip and the pH value read off. This makes them particularly useful for coloured solutions where any colouration affects all segments.