An-Nasai's place among the six canonical Sunni collections rests on a book he did not originally set out to write as it now stands. His first and larger compilation, al-Sunan al-Kubra, included material of varying strength alongside the soundest reports he had gathered over a career of travel through Khorasan, the Hejaz, Iraq, Syria and, for much of his working life, Egypt. Later in his career he prepared a shorter selection from it, keeping the reports he judged most reliable and setting aside material he considered weaker, and it is this abridgement, generally known as al-Mujtaba, that later scholarship treats as Sunan an-Nasai and ranks among the six.
Critics of hadith transmission have long singled out an-Nasai's standards for narrator reliability as unusually exacting among the compilers of the four Sunan collections, closer in severity to the two Sahihs than to the other Sunan works. That reputation is precisely why an abridgement of his own larger book, rather than a work composed from scratch as the canonical text, was accepted into the six: the selection itself was understood as an act of the same rigorous criticism that built his reputation in the first place.
The book is organised, like the other Sunan works, by legal subject, moving through purification, prayer and the ordinary structure of Islamic legal literature, which is part of why the Sunan collections as a group are described as the jurists' books among the six, more directly oriented to legal use than the topically broader Sahihs.