There are common attributes between Product model in CRM and Material model in S/4, for example: Unit of measure, Description, Tax, Sales organization data etc. And CRM and S/4 Model also have their specific attributes. For example, product category are CRM specific concept in S/4, you maintain material type and material group for a material, and these two attribute do not exist in CRM product.
The picture below illustrates the idea that from business point of there are some common attributes between the two model in CRM and S/4. Of course those common attributes are designed and stored in different database table in CRM and S/4. For example, in CRM, the unit of measure is store in table COMM_PR_UNIT with field UNIT, whereas in S/4, it is stored in MARA with field name MEINS.

The target of model harmonization

(1) for those common attributes, the single persistence table and corresponding read & write API must be used in the application. Take unit of measure for example, now in CRM application, the S/4 API to read and write material unit of measure must be used in the code instead.

(2) for those CRM specific attributes, the code for their manipulation should be kept unchanged.

Achievement after model harmonization

(1) All materials must be created in MM01 and updated in MM02.
(2) In MM01 and MM02, you can only maintain attributes belonging to S/4. The CRM specific attributes are completely transparent and thus invisible from these transactions.
(3) In CRM product Web UI, you can still update those CRM specific attribute. For those attributes which are also existing in S/4 ( for example Unit of measure, Description, Tax, Sales organization data ), they are read only and could only be changed in MM02.