If there's a set of topicality rules to be used, I think having something written down is good -- it doesn't have to be worded with mathematical precision. The Monastery Code can be more like what you would call "guidelines". But having something to refer to, which can be adjusted as needed, will more than likely help more than hurt.

So, to the questions:

1. offtopicroot:
   • spam
   • posts which are not redeemable from topical proximity as noted below
   • posts which are deemed to include farcicle Perl, PerlMonks, or Perlesque topics only to veil a clear intention to troll; this is a judgement call and should be used with extreme reticence
2. section nonapproval: Comments which do not qualify under topical proximity as noted below, but which are social enough to be considered "In the Spirit of the Monastery"^*, insomuch as the Monastery is sometimes as much a social place as it is a technical one.
3. labeled: Computer-related conversations which do not implicate, mention, reference, or otherwise involve Perl; this includes topics which could include use of Perl but are otherwise too general to be considered Perlesque. I would consider these to have good topical proximity to Perl, even if not actually about Perl.
4. severity: I see almost no useful distinction between these latter two categories, but if they must be divided, I've identified where I see a relatively logical (if a bit fuzzy) dividing line.

^*Answering this question gave me an idea I'd like to include here, even though it is solutions-oriented and out of scope of the question, because otherwise I'm likely to forgot to bring it up at the appropriate time. A nice feature to consider adding: The owner, or perhaps a Consideration feature, to reparent a mid-conversation node to the offtopic section, should one come into existence, to effectively trim a conversation down to its Perl-related components, with side conversations relegated to the off-topic section. The corner and edge cases here could quickly become overwhelming (what do do with the descendent nodes), but if someone has a brilliant idea in that space, I thought I'd toss this log onto the fire.