The six books, of which this is one, were not chosen by any council or authority. The grouping emerged over the following centuries as scholars came to treat the same set as the standard reference collections, and even then the sixth place was disputed. Nothing about the process resembles the fixing of a scriptural canon, and the difference matters for reading the atlas correctly.
In Sunni Islam the Quran is scripture and is one text. A hadith collection is a compilation of reports, each carrying its own chain of transmitters and each open to being graded, and a report can sit inside a canonical collection and still be assessed as weak by the very scholars who use the book. The apparatus of criticism runs on the chain rather than on the content: who transmitted from whom, whether they overlapped, whether they were reliable, whether the chain is unbroken. Shia collections are different books gathered by different criteria, so the phrase the six books describes one tradition rather than Islam as a whole. Abu Dawud is a good place to see the machinery because he annotated it, and his grading notes are part of what has been transmitted alongside the reports.
The ranking inside the set is itself a piece of history. The collections of al-Bukhari and Muslim came to be treated as the two soundest, the remaining four as valuable but not of that grade, and the sixth place was contested for a long time between Ibn Majah, the Muwatta of Malik and other candidates. Different regions settled it differently before the present list became conventional, which is a reminder that a canon here is a scholarly habit that hardened rather than a decision somebody made.