Every day more than thousand financial documents are created and posted in sap. To get these transactions posted into sap, we require posting key to make it either a debit entry or credit entry. For recording these transactions, SAP has delivered standard posting keys for use instead of letting you define a new one. Based on the transaction types in sap, these are defined and used.
How about creating a new posting key or changing the existing one for your business. Well, you can do that but be ready for the consequences. All sap standard posting keys are defined and has reference to various tables in sap. If you define a new new key or change the existing standard posting key then you must update the reference table that are related and updated as in the case of standard keys.
In standard posting keys, all field status are kept to optional entry because by making these fields mandatory or required your field status may not work with the filed settings of GL account master as set by using transaction code fs00.
In sub ledger, you do not have separate field status group like it use to be in GL master. Now to control filed status for accounting entry in GL related transactions, it will be easy to manage from gl master assigned filed status group. That is the reason why we have only two key 40 and 50 for general ledger posting in sap.
However in case of sub ledgers, you do not have such concept, as fields for sub ledger accounting entries are mostly managed from its posting keys. To manage different transactions in sub ledgers, SAP has defined different keys based on the transaction we use. You can view list of standard posting keys in sap through transaction code OB41.