The operational ceiling is the maximum altitude at which the helicopter is permitted to operate and nearly coincides with the service ceiling, but might not.

For instance, a pressurised helicopter might have a cabin pressure limit which results in an operational ceiling lower than the service ceiling, something very rare for helicopters.

Service ceiling is the altitude at which the rate of climb (ROC) drops at a specific value, depending on the helicopter type, and a limited range of speeds occurs.

Absolute Ceiling is the altitude at which only one speed is available and the helicopter is no longer able to climb at all. This corresponds to the altitude for which the power required and power available curves are tangential to each other (Power required = Power Available).