What is the difference between Last Due and Last Performed on the PM Master Schedule form?

While a PM job may need to be performed at a certain interval, it realistically may not be completed then. This gap is also what calendar slack notes, as the slack is the number of days after it is due that it has to be performed. 

The scheduling method sets the new Next Due Date after a PM Master has generated a new work order.

Last Due will set the next due date based solely on the frequency count, either in days or months. The completion of the latest work order does not factor into determining when the next work order should be generated

Example: A weekly PM has a schedule method of last due and frequency of 7 days.  This PM Master will generate a work order every 7 days.

Last Performed will set the next due date based on the frequency but only once the latest work order is completed or canceled.  The status of the latest work order does become a factor.

Example: A weekly PM has a schedule method of last preformed and a frequency of 7 days.  The latest work order is scheduled for June 7th but is completed on June 10th.  The next due date is now June 17th rather than June 14th.

NOTE : Using Last Performed increases the risk that a new work order will be generated far outside of the required frequency for the PM. For more information on the advantages and disadvantages, see Last Due vs Last Done.