There are three types of question difficulties: easy, medium, and hard. You can pool questions with similar difficulties together by enabling question pooling from the Question pools settings in the General configurations.
You can specify pool size for every difficulty. The assessment will have questions from every pool based on the specified pool size for every difficulty.
For example, there are 10 questions in an assessment - 3 easy, 3 medium, and 4 hard questions. You enabled question pooling and specified 2 easy, 2 medium, and 2 hard pool sizes. Then the total questions in the assessment would be 6 questions with 2 easy, 2 medium, and 2 hard questions. These questions would be randomly selected from the 3 easy, 3 medium, and 4 hard questions you have added.
The total number of questions in the pool cannot be 0.
The pool size cannot exceed the total number of questions for that difficulty.