Mahendran,
I suggest you to read below WIKI to understand more about your subject question.
What is rescheduling and how does it work - ERP Manufacturing (PP) - SCN Wiki
Whenever the MRP finds a requirement, it checks if this requirement can be covered for an existing fixed receipt (within the rescheduling horizon), before creating a new replenishment element (planned order). This procedure is called “rescheduling check”.
If a fixed replenishment element is found, an exception message is created, suggesting the user to reschedule this fixed element in order to cover the shortage.
BR
KK