Why some 2-way diverters have 2 taps, some 3?

Possom1

Why do some 2-way diverters have two taps/dials and some three?

For example, these are 2-way diverters on the linked-to page below. Some have two taps/dials, some three.

http://canada.hudsonreed.com/bathrooms/shower-valves/2-outlet-valves.html

Tow

Upstairs and downstairs shower hoses.

Possom1

[font=Verdana, Arial, Clean, Helvetica, sans-serif]If you're using such a product between *only* an overhead shower and also a wall-mounted hand shower (nothing else), then am I correct in thinking that the 2-way diverters with 2 dials will allow use of either the overhead or the wall-mounted shower at any one time. Alternatively, if I use a 2-way diverter with 3 dials, both the overhead and wall-mounted showers could be on simultaneously (though water flow would be impeded). [/font]

[font=Verdana, Arial, Clean, Helvetica, sans-serif]If using a 3-dial 2-way diverter, can the wall-mounted shower be switched off so that water is being emitted only from the overhead? [/font]

[font=Verdana, Arial, Clean, Helvetica, sans-serif]Is it especially more difficult to plumb the 2-way diverter with 3 dials?[/font]

K.Flyer

Usually,
One is Hot / Cold blend valve, one is flow valve and the third is a diverter to over head rain shower / smaller shower / body jets.