Rules of engagement
In military practice, rules of engagement (ROE) define the situations under which a unit or individual can open fire on a suspected enemy, without receiving direct orders from higher commanders. While the term probably originated in the US, it is reasonably well standardized in NATO, Commonwealth countries such as Australia and New Zealand, and allies such as Japan and Singapore. ROE are usually classified information, for the obvious reason that one does want one's enemy to know how far they can push before being fired upon; that neutralizes the deterrent value of military forces.
They are distinct from the Laws of Land Warfare, operating at a tactical rather than a policy level. A basic principle is that a unit has the right of self-defense, although political or humanitarian considerations may intervene: a unit may be restricted in the directions it may fire, because the fire may land across borders or in civilian areas.
More restrictive rules of engagement variously let a unit fire only when fired upon, while less restrictive rules lets the local commander open fire when he believes he is about to be attacked. In air-to-air combat, a restrictive rule of engagement may require a fighter aircraft, even though it has long-range radar-guided missiles, to fire only on a target that the pilot can confirm hostile by visual observation.
There are very difficult real-world considerations. In one incident in Iraq, while the detailed ROE were not published, they appear to have authorized the use of deadly force against anyone that appeared to be aiming a weapon at them. U.S. troops were under antitank rocket fire from one balcony of a multistory building. From another balcony, a news crew aimed a camera, with a long cylindrical lens, at them. The news crew was immediately taken under lethal fire, because the troops' viewing system did not have the resolution to tell them, literally in seconds, if a long tube pointed at them was a rocket launcher or a camera. Note that they were actively under fire at the time.
In contrast, for some sensitive operations such as strikes by armed drones, there is not simply a list of rules, but a senior officer and a military lawyer must agree that the target is valid.