North American Route Program (NRP) application
 

Can you help me understand the applicability of North American Route Program (NRP)? I have already read Advisory Circular 91-91K, which I assume is the primary publication describing NRP, but still it is not clear to me.

I am wondering whether when you fly at FL290 or above, you have to adhere to NRP rules or is it voluntary?

I am also concerned if it is theoretically possible that the same connection between the same two WPTs/Navaids can be used in accordance with the NRP rules and when you fly random route?

Or maybe you can help me find the right person in the FAA, who can help me with this issue?