Yes, it would be convenient to have all the information about sub-sampling and skips in one place, but I don't think we have a convenient location for that on the website. If you see a lot of NA (".") codes, the first place to look is at the questionnaire (in an appendix of the main report), find the relevant question, and then back up to see who should answer it. The second place to look would be at the description of the survey, usually in the first chapter of the report.

Also you can look for variables with a title that includes "eligible" or "eligibility". These will help you figure out any subsampling. When there is subsampling, it usually means something like taking every person in every second or third household.