# When may one post responses that aren't direct answers?

Recently, this question was asked on math.SE. I had originally thought the subject was the definition of the modulus $| \cdot | : \mathbb{C} \to \mathbb{R^+}$, so I posted the shortcut for solving the equation as a comment so as not to be accused of not answering the question as has happened to others before.

When it was posted as an answer, I was surprised that didn't happen; at the risk of sounding petty, I somewhat regret fearing downvotes and not doing so myself, given the answer's overwhelming (favorable) reception. Upon reading the original post and first answers again, however, it seems not to match what the asker intended; though useful to know, such a reply doesn't really address the question itself. (I may very well be wrong about this, but that was my impression.)

When is it acceptable to post information or thoughts that are only related to a question rather than direct answers? I very much enjoy this site and would like to post more often, but it's difficult to get started here or anywhere when you keep holding back until you're confident both that your answer is correct and that it won't be considered inappropriate or not what one was looking for.