One that can drive the actuator on its own - a raised boom, a suspended weight, a machine on a slope. It tries to move the cylinder rather than resisting it, so without something holding it back it descends at whatever rate gravity and the return line allow rather than at the rate commanded.

Because a hose or pipe failure between the valve and the cylinder would leave nothing holding the load. Mounting directly on the cylinder port means the load stays held whatever happens downstream, which is what makes these components a safety item rather than a control refinement.

Holding means the load does not move at all in neutral and does not creep over time. Controlling means that when the load is deliberately lowered it descends at the commanded rate rather than running away. Different valves in this group answer those two requirements, and some systems need both.

Not reliably. Spool valves have internal leakage past the spool by design, so a load held only by a closed spool will creep down over minutes or hours. For any load that must stay put, a dedicated holding valve with a positive seat is required.

No - a relief valve limits maximum pressure and does nothing to hold a load in position. The two functions are unrelated, and assuming a relief valve provides any holding capability is a misunderstanding that shows up as a load slowly descending with everything apparently set correctly.

Set too low, the load creeps or descends uncontrolled. Set too high, the system cannot lower the load at all or does so jerkily, and pressures rise unnecessarily. Setting is done against the actual load, and a valve set on the bench to a nominal figure is rarely correct on the machine.

On lifting equipment and machinery with suspended loads, holding arrangements are generally required by the applicable machinery safety standards, and the mounting requirement forms part of that. The precise obligation follows the equipment type and jurisdiction, and it should be confirmed rather than assumed.