In Hadith terminology, a Musnad is organized by the original narrator (the companion who reported it), regardless of the topic. A Sunan , on the other hand, is organized topically according to chapters of Islamic law (Fiqh), such as purification, prayer, and business transactions.
Serious students can compare al-Darimi’s narrations with those found in Sahih al-Bukhari and Sahih Muslim to find unique details or wording variants. sunan darimi english pdf
