A manned aircraft is a conventional aircraft with people on board: a helicopter, a light aircraft, an airliner, an air-ambulance or a glider. Unlike a drone, its failure or a collision puts human lives in the air directly at risk.
In the open category a drone must always give way to manned aircraft — when one approaches, land or descend. The reasons are simple: manned aircraft are faster, they may fly low for reasons the drone pilot on the ground cannot see, and their pilots may not spot a small drone in time at all.