Refer to figures.
As altitude increases:

• The power required line moves up and expands to the right and
• The power available line moves down.

Where there is an excess of power available over power required, a helicopter will be able to climb, until the point at which the rate of climb (ROC) drops at a specific value depending on the helicopter type.

This specific value of ROC determines the altitude of the "service ceiling" and a limited range of speeds occurs (between V_MIN and V_MAX).

If the helicopter flies below the V_MIN or beyond the V_MAX, it will be unable to maintain its altitude.

In case the helicopter conducts a level horizontal flight at its "absolute ceiling", where the power required and power available curves are tangential to each other (Power required = Power Available) and the rate of climb is zero, then there will be only one speed for straight and level flight, i.e V_MIN will coincide with V_MAX.

For a Twin Engine Turbine Helicopter (TETH), according to CAP 758 Figures 3.15 and 3.16, the service ceiling for both All Engines Operating and One Engine Inoperative is the altitude where the margin between power available and power required permits a rate of climb not more than 150 ft/min flying with V_Y(top left corner of each graph).