A shower valve is that part of the shower which controls the flow and mix of hot and cold water into the rest of your shower. It performs this using a combination of diverters, flow controls as well as temperature controls.
All valves usually have a single temperature control. The temperature control varies the proportion of the hot and cold water that is mixed allowing the heat of the resultant mix to be controlled. Once the water has been mixed in this part of the valve various flow controls and diverters then direct the mixed water to the different functions of the shower.
A flow control is simply a valve controlled by a handle or lever on the valve. It takes water that has already been mixed by the valve and restricts its flow from full off to full on allowing fine control of the rate of flow between these two extremes. Each flow control controls the rate of flow from a single valve outlet and from there to a single shower function. To control a shower with two functions such as an overhead shower and a hand-held shower one option is to use a shower with two flow controls. Each flow control independently controls the rate of flow to a single outlet; this means that you can have both controls on at the same time, this is in contrast to diverters.
A diverter is a switch used to direct water that has already been mixed in the valve to one of two or more valve outlets. Besides a diverter has an off position that cuts the supply of water to all the showers outlets. Diverters do not supply water independently to the shower outlets; only one can be on at a time. Also, diverters do not allow fine control over the rate of flow, although there may be some control each outlet is full on or full off.
Sequential Shower Valve Control
Referring to Review of best shower heads, we can say, that the most showers have at least two controls, a temperature control as well as flow control diverters, nevertheless, a sequential valve has a distinct lifter control combining temperature as well as flow control regulation. As soon as a subsequent control is turned the flow to the shower is turned full on the cold as the handle is turned to its full extent the flow remains in full flow, but the temperature is increased to its maximum. With a sequential valve the temperature con be controlled finely but not the flow. Loads of companies out there are making shower heads but only a few worth checking out, I suggest one mentioned above along with Showerheadly.com
Thermostatic shower and Manual Shower Valves
Thermostatic shower valves consist of a built-in anti-scald safety appliance that discontinue the flow of water in any situation where the heat of the mixed water exceeds a (usually) pre-set value. Manual valves do not provide this safety feature. Some thermostatic valves also allow the water temperature to be set and maintained.
You might then have the flow control feeding the overhead shower and the diverter switching between the body jets and the hand-held. That means you could have each of them on separately, or you could have the overhead shower on with hand-held shower or the overhead shower on with the body jets but not the body jets on with the hand-held. But how you have this plumbed-in is your choice, you just need to make it clear to your plumber.