Need clarification for MRP Run

Dear SAP Guru,

Need the clarification for MRP Run:

1.At the time of MRP Run what is the status function for - shipping notification?

2.First i have run the mrp planning mode is normal after that i have some changes in material master , so planning mode is - RE Explode planning & Bom ? AT THE TIME OF MRP RUN  it is correct

3.In background MRP Run at MDBT - what is the PLANNING MODE ?

4. How will work on automatic PO Create through MRP Run PR\S.?

Kindly give me the feedback , i am waiting for your reply.

Thanks,

Uma