What is the main difference between Flow regulator valve and flow Restrictor valve in terms of flow and pressure?

When a liquid is passed through an orifice, there will be a drop in pressure & increase in Velocity(Kinetic Energy). But the flowrate @ inlet and outlet would be same. In this way pressure can be controlled keeping the flow constant.

Please device symbol. Because i see very different regulators and restrictor valves . for exemple 2 line , 3 line , pressure or  heat compensator restrictors. 

La soupape de regulation debit(c est regulation de vitesse d un recepteur)

La soupape de restriction pression(c est regulation de pression du circuit hydraulique)

 By flow regulator we can control the flow but. the flow rate is the ratio between the pressure gradient between inlet and outlet and the fluidic resistance of the device. To get a constant flow, the fluidic resistance shall vary linearly with the pressure gradient. 

Pressure control valves comprise a mechanism that opens the valve above a predetermined pressure threshold, the pressure at the outlet remaining substantially constant while the flow increases dramatically after valve opening. Almost all passive valves on the market or valves in the human body (cardiac valves...) are based on this principle.

I've seen that a pressure regulator regulates the pressure from upstream to required pressure downstream and that a flow regulator regulates the flowrate, obviously.

But, theoretically, doesn't one regulate the pressure to induce a specific flowrate? what is the actual difference between these two devices (in theory or in practice)?

I've also seen that flow regualtors are more predominant in liquid applications and pressure regulators are more common in gas applications. Compressibility of a gas, I would assume, makes it more difficult to regulate the flow of a gas compared to liquid.