Because PAPI is a system with a focused, projected, and ‘sharp transition’ beam, the limitations of its use depend more on the airfield installation, aircraft type, and its purpose.
Theoretically it can be used as low as it is possible to see the lights, depending on weather and task. Thus for a visual / night approach in a helicopter, PAPI could provide guidance all the way down to the pad (the PAPI boxes could be laterally angled towards the centreline).
In practical terms the height bands defined by the vertical angular difference in the colour changeover, which change with altitude – tightening as altitude reduces, dictate usability together with the aircraft speed and control response.
Airfield (runway) use depends to some extent on the range of tasks.
The discussion of MEHT relates to the centre of the beam, but might overlook a small range in height depending on the angular difference in the point of colour changeover. Adjusting these settings (within limits) could provide acceptable accuracy for a greater range of aircraft types. There is of course a point where larger aircraft would not meet the safety requirements and an alternative PAPI set would be required.
There is also some variability in an installation’s lateral displacement, angular setting, and a particular design's angular beam spread.
These aspects focus predominantly on the safety clearances at the threshold, but in low vis the system is used more a (spot) check of the primary guidance, opposed to a full guidance system. These checks (a quick glance) can be as low as it is practical to take time to look and the range of visibilities, e.g. one or two spot checks after Cat 2 visual contact.
For non-precision low vis approaches, the PAPI could become the primary guidance system after visual contact, and the system could again be used as low as practical. However, there should be a gradual transition from using PAPI for guidance to that of a spot check of the final visual approach and landing.
Changing the glideslope angle also affects the range of height between colour changeovers, thus a steep approach using standard changeover settings might initially appear easier to fly, but at lower altitudes it provides more viable – flyable guidance.
PAPI provides a highly accurate beam defining the glidepath, with low ambiguity in deviation relative to the beam centre than the older systems (VASI). It is important to consider how the system should be used, in what situations, and aircraft type. It is not necessary to have a hard cut-off / min-use height, but crews should have knowledge of how the system works and the difference between a guidance system and an aid to check / monitor the late stages of the approach.