So how could you possibly even get into hot waters, no matter what might be considered common decency?
Okay,let's take one example. I come from a pretty hard-core geek background and worked for a while with a small group of geeks in software development. We were a mono-culture. Meetings used an extremely efficient unwritten code. When you thought you understood a point someone was making and you either understood or disagreed, you cut them off, and raised your voice slightly. They instantly evaluated whether they thought you had understood them and either continued, raising their voice, or ceded control of the conversation. This could go back and forth for a second or two, with both sides evaluating the strength of their point and the other's point until someone backed down (which usually took maybe 2-3 seconds at most. Failing to cede to a stronger point made you look like an idiot, so there was enough incentive to realistically evaluate.)
As long as everyone was understood this verbal ballet, you could have a 2 hour meeting with barely a single sentence carried to completion. It was an intricate dance with everyone participating, barely a wasted word, and it would also last about 30 minutes instead of the two hours.
It also utterly excluded anyone who wasn't raised middle-class white male geek. The idea that interrupting people was horribly rude, that people who had something vital to say would never speak up because they were raised to not interrupt didn't occur to anyone in the group. The idea that people felt dismissed, demeaned and generally devalued wasn't even on my radar.
Now in this case, no one went to HR. What it took was for me to get ever so slightly annoyed that one of our best programmers (a woman) let us waste half an hour walking into the swamp that she was well aware of. She explained (with far more patience that I would have shown if the roles were reversed) why she did not feel comfortable speaking. She'd never been asked for her opinion. She was usually cut off after a sentence or two, etc., etc.
From my cultural background, it was *her* responsibility to barge in, interrupt us, and if we didn't catch on immediately, raise her voice to indicate this was a "real" point.
Anyway, we did get straightened out, adjusted our meeting culture to take into account the reality that a modern high-tech workforce is probably only 20% standard middle-class white male geek, and that the cultural standards that I assumed were default were anything but.
A code of conduct would have acted as a reminder that one doesn't have to be a jerk to end up being a jerk...