- If I had enough reputation : by adding in the question a comment? an answer written like a question? Something else ?
- If I don't have enough reputation to comment or edit out of my questions : how ? Then, how do I link to the parent question ?
In this case, you're really asking a different question.
"What is X for?" exists.
Your question is, "Does X achieve A?"
If that's not answered in the more general question, then you usually have a separate question, and it's fine to post it separately. You can link to the related question if you judge it would be helpful.