If all you need is simple clarification on an answer (e.g. "does this advice apply to Linux and Windows, or just Linux?") then add a comment on the answer and the poster will almost always reply. Good answerers will edit their answer to include the missing info, and will let you know about the edits in a comment reply.
Also, if I think part of an answer is incorrect or unclear, I'll usually comment on the answer to give the answerer an opportunity to correct his mistake.
Note that comments are intentionally simplistic: no formatting, limited length, no threading, etc. This is to force users to edit and improve their questions and answers instead of piling on long discussion threads which require a lot of work for future readers to digest.
If, however, most of the following are true, I generally ask a new question:
the additional info I'm looking for isn't even mentioned in my original question, but is something I thought of after reading the answer(s) to my original question
the additional info I'm looking for makes sense as a standalone answer, e.g. someone coming in from Google would find it useful even without seeing my original question
the answer is an accurate answer to the question I asked, and adding the additional info (which may be specific to my particular situation) may make the Q&A pair more confusing and less helpful to future readers
it's been a long time (several weeks or months) since I asked the question and got some answers, meaning that the original answerer may not be watching out carefully for replies anymore
I've already accepted the answer as correct (it's harder to get answerers interested in answering a question if you've already told them that you got the answer to your question!)
Use your judgement about borderline cases, and feel free to look at other questions for guidance and to see the pattern.
As Simeon notes above, you'll want to link back to your original question inside your new question. e.g.
In my original question here
(hyperlink "here"), I asked
(1-sentence summary). While trying the
accepted answer, it brought up an
additional question: what happens if
(your new question here)
BTW, Simeon above isn't following Answers' etiquette correctly-- you shouldn't answer a question inside a comment. 🙂
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