For Geoffroi de Charny, writing around 1350, there were three elements to the law of arms.
The first was custom: what did experienced men at arms think would be expected, absent an explicit offer or agreement otherwise?
The second was explicit agreements or promises: such as the mention of specific rules in the announcement of a particular joust. In the 14th century these left so much unsaid that Charny must have expected customary precedent to do most of the heavy lifting, and that an experienced jouster would have a mental template of more or less how a formally announced joust would be run.
But just like judges of Common Law today, Charny's judges would often encounter disputes where neither unwritten precedent nor written rules gave a clear answer, and they would be forced to decide by a combination of analogy and their own intuition of what was just. This would then become part of the body of precedent used by future judges.
Interestingly, the branch of modern law most like Charny's law of arms may well be the law of war, which is still strongly rooted in customary norms.