Why does the Hourly Attendance Report for Positive Attendance Schools show a plus sign for some periods?

A plus sign (+) indicates that the student attended the period and will generate ADA but there is a code other than "P" for Present in the student's period attendance field. 

For example, if a student is late to Period 2, a "T" for Tardy in their Period 2 class would generate a plus sign in the Period 2 field on the Hourly Attendance Report.