A fall from > 1m height above the ground is already considered a fall. In most cases this is prevented by an ordinary railing, be it on a staircase (the side handrail), on a building pit, on scaffolding, etc.
When working on roofs, on the other hand, you have to secure yourself with a fall protection device, a kind of climbing harness, because there is no railing. The fall protection must be approved and must not have exceeded the expiry date. Always check beforehand whether technical or organisational measures are preferable to the use of PPE.