Shift factor in maintenance plan and strategy refers to that of,if an order of a equipment is pre ponded or post ponded this comes to effect.for example if the maintenance order is executed earlier then it is considered as earlier completion and if the maintenance work order is executed later than that of it's date then it considered as later completion.
In sap they have this option of shift factor of earlier completion and shift factor for later completion for the very purpose.
Completion requirement wont allow the next order of the particular equipment to be released unless the first one is completed and i am think it has no connection with shift factors.