I often get locked into comment stalemates trying to get specific conditions on open-ended questions so that there is a clear answer or two to address the question. The problem arises when posters fail to single in on the issue, so it's totally unclear whether they have a specific problem in mind and they won't divulge the details or if they have an unfocused, non-specific problem in mind for which there are too many conditions/assumptions involved to even offer a useful answer.
"Lack of focus" is intended to refer to posts that pose multiple different questions. Just take some care, perhaps in a friendly comment, that you don't suggest that the OP break a badly formulated post into a zillion smaller bad posts!
"Needs more details or clarity" refers to posts that cannot be uniquely understood without additional information. Recognizing this might take experience and creativity: many times the poster cannot even tell that their question is ambiguous because their knowledge is limited. Thus, posting an expansive comment can usually be helpful.
There are actually 3 types of problems here:
Needs more details example:
Who is right?
T͟h͟i͟s͟ paper contradicts what T͟h͟i͟s͟ paper says.
Both are from reputable authors. Which one is right?
To understand the question, people have to read both papers before they can even guess what the OP perceives as a contradiction.
The relevant parts of the papers should have been quoted inline, and the actual contradiction explicitly stated in both the Title and the Body.
Needs more clarity example:
A massive wall of text resembling a James Joyce novel (Ulysses).
All this does is demonstrate the OP's total lack of understanding of the concept of paragraphs, and the OP's being unable to communicate organized thoughts.
All the necessary information might be there, but no reader is going to bother spending an hour trying to analyze and organize the question so that it can be understood by anyone other than its author.
Lack of focus example:
I read that …, so I'm wondering, Question B?
And either way, does that mean Question C? or perhaps Question D?
I also don't understand Question E?
The Body contains 4 completely different questions, none of which match the question in the Title.
Assuming you have left a helpful comment which your question suggests you have then sweating about which reason to give is not necessary. I would go for lack of detail/clarity myself whenever I am in doubt partly because I am never quite sure how to measure lack of focus.
If three people vote to close for three different reasons it still gets closed. I did ask a question about unanimity ages ago Does unanimity of reasons to close matter?. The consensus was it does not matter.
Detail versus focus
Focus relates to the 'focal point' in physics, a central point where all light beams converge. If there is no or little focus then there is not such a central point.
A request for more focus occurs when a post is dealing with too many details and questions all at once.
A request for more details is the opposite and occurs when a post has no detail at all.
In the image below the top left corner has low detail and the bottom right corner has no focus. Moving towards the upper right or bottom left corner is adding more detail/focus.
Detail versus clarity
There is a subtle difference between detail and clarity. In both cases a question is not very easy to understand.
Detail relates more to sharpness and the amount of small pieces of information. For example, a very short post with no background information has little detail.
Clarity relates more to the quality and whether the information is well represented. For example a post written with bad grammar and spelling, typos and non logical sentences is not clear.