Posting Keys area helping additionally to control the transaction entries in SAP ERP. You could have several posting keys for the same type of sub-ledger which will help you to imply different controls on postings made from different transaction codes.
IMG Path: Financial Accounting (New) -> Financial Accounting Global Settings -> Document -> Define Posting Keys
When you open the transaction, you could either create new posting key by click on New Entries Button or edit already created one by double click.
If you want to create new one, following pop-up will ask you to define the posting key and the name.
Maintain the options as you are required to do.
You are also able to define field status settings based on the particular posting key.
These are the same groups as in the definition of the field status variant and related field status groups.