Product group is the third classification term for parts, and is mainly intended to assist you with default coding for customer orders and purchase orders. Secondary use of the product group is Discount Categories, and to use them as classification / selection terms in lists of purchasing and sales statistics.
If you click on the button next to the Product group field you can see the part's links to the product group, product group matrix, and coding. These links are shown under the tabs Sales, Purchase and Subc. in the window that opens.
The information shown under the Subc. tab is determined by the setting Subcontract coding via product group on... under the Accounts Payable under the Purchase tab. If the setting is set to Main part you will see the same information as under the Purchase tab.
You create product groups in the Product Groups procedure under Tables in the Global Settings module, and in that procedure you enter which income or expense account should be used by default for each customer or supplier during order registration.