Final answer:
The posting key in FI documents controls the account type, account group, and whether the line item is posted as a debit or credit.
Step-by-step explanation:
Within the line items of FI documents, the posting key controls several aspects. Firstly, it determines the account type to which the line item can be posted. For example, a posting key of 01 is used for posting to a general ledger account, while a key of 11 is used for posting to a customer account. Secondly, the posting key determines the account group to which the line item can be posted. This helps in classifying the line item based on its nature, such as assets, liabilities, or expenses. Finally, the posting key determines whether the line item is posted as a debit or credit. For example, a key of 40 indicates a debit posting, while a key of 50 indicates a credit posting.