The Sunan of Abu Dawud is arranged by legal topic, purification, prayer, marriage, sale, punishment, and it selects for what a jurist needs rather than for what a critic will certify. A letter attributed to him and addressed to the scholars of Mecca explains the principle: he gathered what people were actually arguing from, kept nothing he considered abandoned by everyone, and where a report was weak he said so rather than dropping it, so that a reader could see the whole of the material a legal position rested on and judge it.
That decision makes the book a different kind of document from a collection assembled by the strictest chain criteria. It preserves reports the sterner compilers excluded, and it preserves them labelled, which means it records not only what was transmitted but what a ninth century lawyer thought was usable. Ancient legal practice is visible in it in a way a purely critical collection cannot show. He worked in Basra, travelled the standard circuit of Iraq, the Hijaz, Syria, Egypt and Khurasan to collect, and the book was recognised early enough that it was being taught within his own lifetime.
The arrangement itself carries information. Chapters exist for the topics a jurist has to decide and not for the topics a theologian would raise, so the shape of the book is a map of what ninth century legal practice actually covered, from the details of ablution through marriage and sale to the division of spoils. Reading the table of contents is a quick way of seeing what a Muslim court in that period expected to be asked about.