Sorry I was confused tonight about the W in front of the message
Usually the W stands for warning, but here it is just part of the message class.
So all messages are errors and prevent archiving of a material
Please, if you try to archive materials, then get all people together who use material in their processes. This is the normal way an archiving project is setup.
As said yesterday, the PP experts (consultants or even key users) usually know the transactions for this simulation planned orders, and yes they are stored in the same table and have probably an indicator in one of the fields to enable MD04 not to display them. You may even find the user who created those records from field PUSER in table PLAF and talk to him directly to get a transaction for the display.
You may have to debug the archiving program to know when this message W4031 is issued, as you have material with this message it should be possible for you or an ABAPer, I browsed a few check programs but could not locate the program with this message. Eventually you have to write to SAP to elaborate on this error message, convince them that the note is to vague.
You can call the function module directly from SE37 to investigate a single case.
Product group assignment is probably maintained via MC86, but this is also a topic of your PP consultant.