I found these interpretations from OHSA to be enlightening:
Question: Are safety platforms attached to the forks of a forklift considered aerial lifts?
Reply: No, Aerial devices (aerial lifts) are covered under §1910.67, Vehicle-mounted Elevating and Rotating Work Platforms. An aerial device is any vehicle-mounted device that telescopes, articulates or both, which is used to position personnel. A platform attached to the forks of a forklift would not be considered an aerial lift.
Question: Are workers required to wear fall protection (full body harness and lanyard) when they are up in a safety platform that has a standard guardrail, mid-rail and toe plate?
Reply: No, personal fall protection equipment would not be required when a guardrail system is installed along the open sides of the safety platform.
https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24260