The so-called Yellow Card, which contains the standing instructions for soldiers in Northern Ireland as to when they can open fire, was revised four times between September 1969 and November 1971, and twice more in July and November 1972, the tribunal was told yesterday.
Mr Christopher Clarke QC, counsel for the tribunal, said the instructions were approved at the highest, that is Cabinet, level.
He said: "It is one of the hallmarks of parliamentary democracy that the armed forces act in aid of the civil power: they do not supplant it. So far as the use of force in self-defence or in the prevention of crime or disorder is concerned, soldiers are subject to the same law as any other citizen.
"They may use such force as is reasonable in all the circumstances, but not more than is necessary."
However, the common law, which provided that criterion, did not attempt to define what was reasonable and what force was more than necessary "in the infinite variety of circumstances to which that criterion is to be applied", he said.
Soldiers in the field needed some more specific rules than that, and the rules at the time of Bloody Sunday were contained in the Yellow Card.
The card was not a source of law. It was an attempt to provide a guide in a manner intelligible to those who might have to decide whether or not to fire lethal weapons as to whether they could do so.
A paragraph in the Yellow Card dealing with firing without warning appeared to relate only to firearms, grenades, nailbombs or gelignite-type bombs, but not petrol bombs.
Mr Clarke noted that three soldiers who fired on Bloody Sunday, on their own evidence, shot at one or two people who were crawling along the east side of Rossville Street towards the door of the flats.
On their evidence, the person or persons whom they shot at was either holding a rifle or passing it forward.
Assuming those facts to be true, counsel said, the question arose as to whether the victim could be said to be about to use a firearm for offensive purposes within the terms of the Yellow Card, and whether shooting him in such circumstances was or was not legitimate.