Hello,
If you are referring to the Reference number field then BC will assign a reference number for Prod. Order suggestions:
The reason is for full MRP/MPS planning these planning suggestions are used for other suggestions such as purchases. You can see below my Prod. Order I have a purchase recommendation for 11 units. If I select that line and then select Item Availabity by > Event
You can see one of the suggestions is a planning level suggestion related to that Prod. Order Reference No.:
While I haven't created an actual Planned Order yet that Prod. Order reference number is still be using for planning suggestions.
Hopefully that helps!