Since I've recently noticed, for the first time, that some higher ranking users have the habit of deleting discussion in the form of comments in bulk, under the justification that it falls under the term "extended discussion", I would like to ask - can someone provide me clarification on the term "extended" and what is considered "extended" (as in "prolonged" I guess) and what is considered an "ordinary" discussion?
Just so it is not missed, I'm extremelly pissed (I picked this word very carefully) by this practice for I was under the impression that that is something one does not do in a community where expressing one's opinions is encouraged.
Comments aren't for discussion, per se, they are for clarification, examination, and refinement of the parent post.
An "extended" discussion for the purposes of migration to chat is at least 3-4 back and forth responses between two and only two users. That is, a total of 6 or 8 messages between two users with no intervening comments from other users.
Stack Exchange now automatically suggests moving the conversation to chat when some threshold for "extended discussion" is reached. I'm not sure what that threshold is, but it seems to be around six comments between two people. We can take that as the definition of an extended discussion.
In general, discussion (ordinary and otherwise) should happen in chat and answers should contain complete information. Comments can be used to enhance answers, but they're not a substitute for chat or forum threads.
There are cases where comments should not used, such as:
You don't agree with a given answer, and you keep debating with the OP about that.
If you don't agree with the answer, you can add a comment explaining why you don't agree with what reported; the OP could report you didn't understand what said in the answer, but an extended discussion about that is done in the wrong place, if it is done in the comments. If you completely disagree with the answer, you can answer the question yourself, reporting exactly why you would not do what reported in an existing answer.
You are the user who asked the question, and you report the solution doesn't work for you because something you didn't say in the question; the OP adds more details in the answer, and then you report that even the added solution doesn't work because something else you didn't explain in the question.
It should be better to write a more complete question, where all the significant details of the question are explained. The reason is that adding details as comments of one of the answers would force who read the question to read all the answers and all the comments to completely understand the question being asked; this means the question would be less useful for the users that will read it. It should be reminded that a question and its answers are not for only being used from the user who asked the question, but from any users who read the question.
You are the user who asked the question, and you ask secondary questions about the answer given, in comments.
If the purpose is to verify you understood what reported, that is fine, but if the question is asked because you are having difficulties on applying the suggested solution, then those comments should probably be a different question.