As noted by Oleksandr above the off-topic label is is catch-all for any of the site-defined close reasons or Other if used. Please reference:
Oleksandr also notes that the chosen close reason includes easily found in the documentation which is a rather subjective measure, and it is sometimes used to close questions that seem too easy or show lack of effort.
Unfortunately (in my opinion) people often mark recent questions as easily found in the documentation which once were answered, rather than marking the question as a duplicate which is the correct action for several reasons:
- It directs the user to existing questions and (hopefully) answers
- It creates a search entry point to find the existing Q&A
- It helps to indicate the relative frequency with which a question is asked. High frequency questions may be tagged with faq and may justify writing a canonical answer or inclusion in the pitfalls list.
@ChrisJJ You seem to misunderstand. Although in the case of your questions duplicate is a better designation, there are valid reasons to close questions. Each site can specify some of the more common ones. No matter what these are the Stack Exchange software labels all of them as off-topic. So for example we have determined that being "a simple mistake or ... easily found in the documentation" is a valid reason to close a question; this is forced under the category of "off-topic" by the Stack Exchange software, even if the question is clearly about Mathematica.
Related questions that I think you should read:
e.g. "Having a question put on hold is neither a disgrace nor a punishment. It is simply a matter site house-keeping."