Fact less Fact table does not contain any facts. Generally Fact less fact tables are used to record the events such as students attendance, attendance of participants for an Event like a Meeting. Actually this is not a Fact table but due its position we should call it as Fact table without facts.. Look at the following image..
See the attendance Table in the Image, There is no measure or fact but Attendance Table sits like a Fact table and also have the connections to dimension tables and hence we can say that it is Fact Table and since it does not contain any measures so that we can call it as fact less fact table.
Now Let us understand the use of the Attendance Fact less Fact table....
If a student attended a class then we enter a Record in Attendance table else no row will be stored. Only aggregation here is possible only count