$29$ CFR $1926\mathrm{.}501($b$)(1)$ states that $$Each employee on a walking$/$working surface $($horizontal and vertical surface$)$ with an unprotected side or edge which is $6$ feet $(1\mathrm{.}8$ m$)$ or more above a lower level shall be protected from falling by the use of guardrail systems, safety net systems, or personal fall arrest systems.$$

For comparison purposes, the following are several different OSHA fall protection requirements:

General Industry $-4$ feet

Construction $-6$ feet

Scaffolds $-10$ feet

Portable Extension$/$Step Ladders $-$ Maximum working height of the ladder

Fixed Ladders $-20$ feet

Steel Erection $-15$ feet or $30$ feet

Crane Assembly$/$Disassembly $-15$ feet

Research why these requirements are different.