Hi Keerthan,
In brief,
Completion Requirement helps in not triggering a Maintenance Call Object, until the Previous Call Object has been made NOCO (Notification) or TECO (Order.
A Shift Factor, instructs the system that if the Completion Date of the Call Object is different from the Plan Date (Required Start in Notification and Basic Start in Order), shift the plan date of the next Call object accordingly. (According to the % you have mentioned in the Shift Factor for Early Completion and Late Completion fields)
Jogeswara Rao K