It's up to interpretation, but I think for melee units, most common interpretation has been noble vs noble combat. For ranged units wounding others, I think interpretations have been more evenly split between a noble and the unit wounding. Either way, as long as there is some mutual cooperation folks shouldn't care too much on this either way.
Well, heroes are said to be distinguished by actually involving themselves in combat directly, unlike the rest.
That doesn't always seem very plausible, though.
