What are posting keys and what is the purpose of defi ning them?

Posting keys determine whether a line item entry is a debit or a credit, as well as
the possible fi eld status for the transaction. Posting keys are delivered in the SAP
solution. If you want to change posting keys, such as making additional fi elds
optional on payment type, the best possible action is to copy the posting key that
needs to be modifi ed and then modify it. Figure 1.24 shows the standard posting
keys in SAP solutions.