Boom lift: harness and lanyard, to the machine
A boom platform is on the end of a long lever. A sudden boom movement, a hit from another machine, or dropping a wheel into a hole can catapult an occupant clear of the basket entirely, over the guardrail and not through it. A guardrail cannot prevent that; a lanyard anchored to the machine can.
The anchor is the manufacturer's designated point inside the basket. Not the guardrail, and never a nearby structure.
Scissor lift: the guardrails are the system
A scissor lift goes straight up on a stable base and does not whip. With rails up and the gate closed it is a guarded working platform, and the guardrail system is the fall protection. A harness is generally not required.
The corollary matters more than the rule: the guardrails only work if they are up and the gate is shut. Climbing on the rail, or leaving the gate chained open because it is convenient, removes the entire protection.
How to brief it so it sticks
The version crews remember is: if the platform can swing, tie off. If it can only go up and down, close the gate.
Site rules can be stricter than the regulation, and some general contractors mandate harnesses in everything. That is their prerogative, but a blanket rule is worth explaining rather than just imposing, because a crew that does not understand why will apply it inconsistently on the machine where it counts.