What is the difference between Sunan of Abu Dawud and Maraseel of Abu Dawud?
I heard that Maraseel of Abu Dawud is not actually a part of Sunan of Abu Dawud.
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The Sunan and the Marāsīl are both authored by Imam Abū Dāwūd rahimahullah.
In essence, a compendium of Hadīth called a ‘Sunan’ is known in the terminology of the Muhaddithūn as a collection of Ahādīth related to rulings (ahkām) compiled in a juristic sequence (Abwāb al-Fiqh). Thus, the first chapter is commonly the Laws of Purity followed by the Laws of Salāh etc.
Imam Abū Dāwūd rahimahullah assembled his book on Sunan from approximately 500,000 Ahādīth. He highlights any Hadīth which is weak in strength as well as commenting on the narrators of the Hadīth. His Sunan is among the famous six authentic books in books of Ahādīth.
Books of Marāsīl are collections of Mursal Ahādīth. According to the Muhaddithūn, a Mursal Hadīth is a narration in which a Tabi’ī omits his teacher and directly narrates from the Prophet salallahu alayhi wasallam. So books of Marāsīl contain all such narrations where a Tabi’ī directly narrates from the Prophet salallahu alayhi wasallam whilst omitting his/her teacher.
Imam Abū Dāwūd’s book on Marāsīl has approximately 544 narrations. He has compiled this also in a juristic sequence (Abwāb al-Fiqh).
And Allah Ta’ālā Knows Best
Mufti Faraz Adam,
 مصادر الحديث ومراجعه
This answer was collected from DarulFiqh.com, which is operated under the supervision of Mufti Faraz ibn Adam al-Mahmudi, the student of world renowned Mufti Ebrahim Desai (Hafizahullah).