Backup in tactical operations means the operation of firearms ready for shooting while performing a duty. It is important for self-defense of involved agents and to prevent escapes. It is done by pointing to a target or shooting. Back up may lead, but is not limited, to suppressive fire.
The purpose of Back up fire is to cover an operation of one or several agents such as entering a room or arresting a suspect or fugitive.
It is important that own agents or other innocent people are not standing, or moving through, the line of fire.