The Latin phrase "sui generis" means something like "unique" or "of its own kind." Durkheim uses the phrase to describe social facts about societies. He argues that social facts about a society are sui generis. He says that they are created by the society and are unique to that society.
Durkheim is arguing that social facts are not things that we can control. Rather, they have been built up by the cumulative experience of all that has come before us. Because of this, they are sui generis. They are unique and they are not things that we can change.
So, to Durkheim, the phrase "sui generis" refers to things that are outside our control. He uses the phrase to refer to social facts about our society that act upon us but which we cannot ourselves change.
According to Emile Durkheim, society existed before any individual was born and would continue to exist even after the individual is gone. This basically meant that, although society is composed of a number of parts with people representing one of the parts, it cannot be defined by reducing it to the sum of parts that interact to create its unique nature.
He explained social facts as “sui generis” meaning social facts are unique to the society that establishes those facts. This asserts that social facts are pre-existing and not a product of the current population. Thus social facts shape the way the present generation behaves or interacts with their society, making their interaction and behavior predictable. Further, social facts cannot be changed by individual alterations, in their belief system or actions.