Generally, the terms rangeability and turndown ratio are associated with control valve sizing and flow measuring devices.
Rangeability defined as the ratio of maximum controllable flow to the minimum controllable flow that a device designed to. So this is a theoretical value before a device installed.
The turndown ratio gives the width of the operational range of a device/valve. It is not required to handle the maximum possible flow. It defined as the ratio of normal operating maximum system flow to the minimum controllable flow. In the case of boilers turndown ratio is the ratio of maximum heat output to the minimum level of heat output at which the boiler will operate controllably.