"What constitutes a personal attack" is a subjective matter and can not be answered in objective terms.
It is very much like pornography ... wasn't it a Supreme Court Justice who said something to the effect that he couldn't define pornography, but he knew it when he saw it. Well, personal attacks are like that.
When it's all said and done, if you're a board moderator, personal attacks are what you consider personal attacks to be. You apply your own definition. If, in the course of doing so, you anger some of the posters on the board, that's part of the job of moderating. If some poster feels you've overstepped your bounds and appeals to management, they will probably uphold your decision. If they don't, they'll address the matter with you, privately, at which time you can accept their judgment, present your justification, or resign your position (there are probably other options, but these are the key ones.)
Obviously, this yields a wide variety of interpretation of "personal attacks" and an equally wide variety of interpretations of the TOU. That is unavoidable in a public forum. There are no absolutes ... much was we might welcome them.
You could write your own set of rules for what you will tolerate on your board (within the limits set by the TOU), but I wouldn't recommend doing so. If you do, you'll find that, over time, your rules will become as incomprehensible as our nation's tax laws.
You are absolutely right! This is a contentious issue. The best resolution (which I do not expect to see) is for moderators to encourage following the spirit of the TOU instead of looking for specific rules to which they can point as justification.
Fred