I think the posting group is a means to be used to find the inventory, receivables or payables account that the outstanding amount of sales order, purchase order, output production, item, service, etc.. will be posted to chart of account that contain G/L account no. you can also explore more using online help - F1, financial mgt manual and navision essential manual.
Posting groups primer.
The General Ledger (G/L) - Account Code in the Chart of Accounts (CofA) is the sum of all the entries in the sub-ledgers and journal posting directly to the G/L.
Sub ledgers include (but not exclusive to): Customer Accounts, Vendor Accounts, Item Accounts (Item No.).
Postings involving the sub ledgers need to be posted indirectly to the G/L. Each sub ledger account has a Posting group, each Posting Group has a related G/L account.
The G/L account referred to above is also known as a "Control Account" that must not be posted to directly unless for some extraordinary reason (not covered here).
An example: From the G/L, the accounts department can say that the balance on the debtors control account = all the open items on the customer accounts.
General Posting groups are used to decide what accounts are to be updated in the G/L depending on the circumstances of the entry.
They are used generally to identify transactions to different markets or of different products and each combination can (but not always) post to a different G/L account as determined by the business owners.
The VAT posting groups are there (for those countries that have VAT) to post to VAT Input and VAT Output Control accounts as well as calculating how much VAT should be calculated on the sales or purchase entry.
All the General Business & Product posting groups are set up in the Posting Setup and the General and Business VAT posting groups in the VAT Posting Setup tables. How these are set up will determine the G/L accounts that are to be updated.
Hope this helps get you started....Colin