Accounting rules teach how to translate everything that happens in a company to an accounting language, that is, debits and credits.
Dynamics NAV has implemented these rules using posting groups, so the system can translate everything to the accounting language and post it to general ledger entries on the fly.
Posting groups are related to master data. When you create a new record in the master data (for instance, you create a new customer), you need to specify which posting group it belongs to.
In the previous image, you can see the existing posting groups for customers. For each posting group, all the columns are filled with an account value. Dynamics NAV uses these accounts to post the general ledger entries anytime a transaction is made with a customer.
You can create as many posting groups as the amount of detailed information you need. In Europe, for example, you have to separate domestic customers, customers from the European Union, and foreign customers...