There is a short discussion of sabhagata in Abhidharmakosha Chapter Two, the Indryas: Dharmas not associated with mind. The concept of a sabhagata must have arisen to explain the question why there are different classes of beings, how these classes of beings are formed, and how they remain as distinct classes or distinct entities?
Vaishesikas, Sarvastivadins and Vaibhashikas are on the right track, they assume that there is a unique substance or a special entity (padartha), which causes the existence of different sabhagata or species of living beings.
The doctrine of a sabhagata is fiercely objected by the Sautrantikas.
Sabhagata is highly interesting although its discussion is very elementary, for example it doesn't consider the further question how the different species or different sabhagata arise, or have arisen, in the wheel of transmigration? We can easily see how the concept of a sabhagata is important in many ways.