Hey Jeff...
The way I have understood it a RP can be ANYTHING that serves the purpose of a RP (mucho clever, hah? )... meaning anything that can help you understand WHERE you are and WHAT you need to do when approaching or passing over that point. Do I need to clarify it needs to be a STEADY/ permanent point and not a parked car or a cat siting on a fence?
Hoped I helped in some way
VaS