6' refers to the surface you are standing on
1926.501(b)(1)
"Unprotected sides and edges." Each employee on a walking/working surface (horizontal and vertical surface) with an unprotected side or edge which is 6 feet (1.8 m) or more above a lower level shall be protected from falling by the use of guardrail systems, safety net systems, or personal fall arrest systems.
There are a dozen other scenarios following that, all referring to the surface.
