Firstly, "honour" is dependent on context. That means that there are always rules sensitive to time period, culture, social class, and other factors. Secondly, it's worth noting that various words for "honour" in various historical contexts are sometimes interchangeable with "reputation". One fights with honour in order to preserve or progress their reputation, in essence making some kind of point or argument about their reason for fighting. This distinguishes a duellist from a soldier; the former is often fighting as a means of social expression (or representing someone else's expression), whereas the latter is fighting as proxy for some kind of authority.

One point of interest comes from late medieval duelling practices. In ancient and medieval Germanic societies, there was a tradition of judicial combat for use when a court of law could not determine guilt or innocence of a defendant. Note that "Germanic" here predates a unified Germanic state, referring to cultures that spoke Germanic languages and/or inherited Germanic ways. As an example, modern Anglo culture might be considered "Germanic" as influenced by Anglo-Saxon, Norse, and Norman settlement.

Historically, these judicial duels have diverse rules according to time, place, social class, and even gender. From the 15th century Holy Roman Empire, we have examples provided by Hans Talhoffer, a master-at-arms in matters of both individual duelling and warfare. His works detail duels with both conventional weapons and weapons expressly designed for the judicial context, such as an unwieldy hybrid between spear and tall shield. Such weapons exist particularly because of the lawful framework that existed around such duels; two private individuals could not agree to a duel and have it as a lawful affair, with duels requiring the approval of a lawful authority. Such approval was, I believe, unlikely to be granted outside the context of unclear guilt or innocence in a lawful trial. In Christian Europe, the winner of the duel would assumed to have God's favour; older, pagan traditions might have invoked the favour of deities as well, but you'll need someone informed on their specifics.

While a modern fist fight in the street might lack an official, lawful framework, many instances of unarmed violence are of social intent rather than mortal intent. There are elements of honour and individual reputation in play that may not necessarily exist in warfare (but sometimes do). In essence, honour rules are about the validity of a fight's outcome in the eyes of observers or even just the combatants; warfare is more severely predicated upon "objective" power (but not always), which is why an ambush is more or less "honourable" (but not always). Note an exception; in ancient Roman martial culture, it was uncommon to use ambush tactics because they were not considered befitting a professional warrior, which at times cost the Romans against forces that observed different forms of honour (including the Germanic tribesfolk mentioned prior).

I suppose one could summarise this by differentiating between "law" and "need". Even if there is no official law to frame individual combat, individuals might adhere to honour rules in order to safeguard the validity of the outcome. In the case of large scale conflict, the outcome takes strong precedence over its validity. As always, though, there are exceptions in both cases. '

This is just a tiny slice of duelling culture, though! Famously, as commonly expressed in modern entertainment media, Japan had a comparable duelling culture to historical Germanic societies, albeit less lawfully observed, especially during the Edo period following the Warring States period. And as is often the case, historical rules and laws will flow into our modern period if they are not arrested or replaced in some respect. So it might be said that our rules of social honour in single combat are the remnants of old Germanic laws dictating judicial engagement.