This is a tracking category for all set index articles. Pages are put in this category by templates such as {{SIA}}, {{mountainindex}} and {{shipindex}}. That is, templates built with the {{dmbox}} meta-template, which does the actual categorization. Note that since 2017, {{given name}}, {{surname}}, and {{nickname}} have also put pages in this category even though such articles resemble disambiguation pages. (See the discussion that brought this about.) Also see Category:All disambiguation pages and Category:All article disambiguation pages for the corresponding lists for disambiguation pages.