What exactly is a "damp-proof installation cable" ?
Seems more of a European thing which is why it's on the Knipex website. It gives an example of NYM cables, copy and pasted from a Google search:
NYM cable (NYM-J/NYM-O) is a standard, versatile sheathed PVC electrical installation cable widely used in Europe for fixed wiring in homes, commercial buildings, and light industry, suitable for power and lighting, installable in/on/under plaster, concrete, brickwork, and in dry/damp/wet areas, but must be UV-protected outdoors and kept out of direct compacted concrete. It features solid or stranded copper conductors with PVC insulation, a grey PVC sheath, and includes a green/yellow earth core (NYM-J) or not (NYM-O).
Key Features & Uses
Versatile Installation: Can be run on surfaces, embedded in walls, or under plaster in dry, damp, or wet conditions.
Durability: Robust PVC construction offers protection for fixed installations.
Safety: Includes a green/yellow earth core (NYM-J) for safety, connecting to sockets, switches, and lights.
Not for All Environments: Cannot be directly embedded in vibrated concrete and requires UV protection if exposed to direct sunlight outdoors.
Voltage Rating: Typically 300/500V.
Types
NYM-J: Includes a green/yellow protective conductor (earth wire).
NYM-O: Does not have a green/yellow core.
Applications
Power and lighting circuits in residential buildings.
Commercial and industrial premises.
Temporary construction site installations.
Connecting appliances to mains supply.