If you select Occurrences, each attendance code entered on a student's record is counted as one absence.
For example, if a student is absent for an entire day and they have 5 classes, the student will have 5 occurrences.

If you select Units, each attendance code is calculated depending on the length of the absence. It will also include tardies if they are included in the attendance calculation.

You can determine how the length is calculated by your attendance business rules and the tardies in attendance codes.