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. Other secondary uses of product groups are Discount Categories, and to use them as classification / selecting terms in lists over purchasing and sales statistics.
You create product groups in Product Groups in the Global Settings module, and in that procedure you enter which income or expense account will be used by default for each customer or supplier during order registration.
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, account/project/yes/mandatory and PC type. 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.