Calendar slack can be used to create a required date for a work order.
For instance, if you have a quarterly PM that requires the equipment to be locked out in order to perform the work, you may want to build in some days for the mechanic to be able to complete the work order.
Example: Quarterly PM is scheduled for June 20th with calendar slack of 5 days. The required date for the work order is June 25th.
The mechanic now has 5 days to complete the preventive maintenance work which can prove beneficial if the production schedule at a plant is constantly changing.