Refer to figure.
As altitude increases:

• The power required line moves up and expands to the right.
• The power available line moves down.
• Thus, the power margin decreases.

The range of speeds that can be flown exists where there is power surplus (power available > power required). The range of speeds has a minimum (point A) and a maximum value (point B), defined by the intersection between the power available and power required curves.

So, because of the movement of the power curves as altitude inceases, the range of speeds (distance between points A and B) that can be flown decreases, until a limited range of speeds occurs. If altitude increases further, the helicopter will reach a point (absolute ceiling), where only one speed can be flown.

If flying below the minimum speed or beyond the maximum speed, where there is power deficit (power available < power required), the helicopter will be unable to maintain its altitude.