How to determine "ground clearance" for a rig?

Off-Road Ranger I

98
Portland, OR, USA
First Name
Toby
Last Name
Erkson
Ham/GMRS Callsign
WSNV844
Service Branch
US Navy -- STG3
I see there are roads labeled for high clearance vehicles. What is the definition of "high clearance"? What are the measuring points to determine ground clearance? Is it from the ground to the lower shock mount or to the bottom of the rear diff or simply find the lowest looking point of your vehicle and measure to the ground?