First off, do a little sanity-check: are you sure the question is off-topic? If it's about cats, then that's an easy check; otherwise, review the faq.
Next, do another sanity-check: is it spam? Is it someone trolling for attention? There are specific flags for "spam" and "it is not welcome in our community" that are a heck of a lot more effective than closing for stuff that needs to be removed ASAP - don't waste your time and everyone else's time trying to find the best close reason for "Cheap Gucci Handbags".
Then, see if any of the close reasons are reasonably close. Not just the OT reasons, but also things like "Too Broad" and "Primarily opinion-based". If OT is the best fit, then read the predefined reasons carefully: the OT reasons listed in the dialog represent the most common off-topic questions, so there's a pretty good chance that one of them will fit, but... Keep in mind that there are also a lot of perfectly good questions that touch on the same topics; how the question is asked can matter almost as much as what it asks about. Consider if an edit might be able to transform the question into something more acceptable.
Finally, either pick one of the predefined off-topic reasons, or - if none of them fits - back up and select "other" on the main flagging dialog. If you do the latter, keep in mind that you're asking a moderator to review the post: is it something that really needs a moderator's attention? If you don't feel strongly about it, don't flag. If you do, great! Explain why. Don't just type "off-topic"; provide enough information to help the moderator understand what problems you envision the post causing.
Oh - and to answer your last question: right now, no one1 sees the off-topic reasons you choose when flagging. That may change in the future, but for now they exist primarily for your benefit, to provide examples of the sorts of things that you should consider flagging when you come across them.
1They are recorded, I'm looking at them in aggregate and will probably expose that information to moderators at some point, but the design is in keeping with the whole "training wheels for future close-voters" aspect, not "feedback for askers" since - no offense - a lot of flaggers have really weird ideas about what is on or off-topic.