How to use the Purchase order Dates for planning in conjuction with the supplier analysis?

In the purchase order several date fields are available, where for planning the most important are:

• Request Date - used within MPS/MRP and defined as needed internally
• Promised Delivery Date - used within Supply & Demand and defined as when the supplier can deliver. Also used to register supplier performance in the Supplier Analysis program, the difference between Promised Delivery Date and the Actual Ship Date.

When both date fields are used as described, how to handle and monitor issues between the MRP messages (internal needed and controlled) versus the visibility of open orders in S&D and supplier performance?