Inventory-Managed Manufacturer Part Number: Stock managing?

Hi to all,

IMG of Inventory-Managed Manufacturer Part Number says:
The Inventory-Managed Manufacturer Part Number component provides functions to identify parts, and administer interchangeability between original and interchangeable parts:
o Parts are found by means of the manufacturer specific details (manufacturer part number (MPN) and external manufacturer), are identified, and are displayed by the System.
o Parts, which are interchangeable with each other in respect of their use in Purchasing, Inventory Management, Material Requirements Planning, and the Availability Check, are held in the System with their own stock. The system supports handling of full and two-way interchangeabilty, as well as mapping of one-way or restricted interchangeability.

According to his, we've followed document Manufacturing Part Number & Approved Manufacturer Part List (AMPL) in SAP MM by Sudhakar Jayaraman (https://archive.sap.com/kmuuid2/106f94ee-8dc2-2a10-a485-882871fa5c11/Approved%20Manufacturer%20Part%20List%20(APML).pdf) to implement it.

But, once the PO is recieved (MIGO), the stock (MMBE) of the HERS type material doesn't rise (the "internal material number" (1308 in the document) does)

So, our question is: Does Inventory-Managed Manufacturer Part Number really hold in the System with their own stock? How? Is something missed in Sudhakar Jayaraman's guide?

Thank you so much in advance for your kind attention!