During a document entry in SAP FI module fields are displayed based on the transaction and the account we use. We customize the account specific field status in company code view of general ledger account master data setting through transaction code FS00.
Posting key field status always used for certain type of fields based on the entry you chose to make.
When you define field status you must consider that fields appeared in one place should tally with the other like for example if a specific field has been set as HIDE in general ledger account master data field status and the same filed is required entry in document type or posting key then you will get an error message.
In general ledger account master you assign the field status group that is required for the expenses account which has a major role in document entry. For example an expenses account should be assigned with a field status variant for which cost center has to be required entry. These field status groups are summarized in one filed status variant which you assign to your company code. Without this an assignment of field status group to your general ledger account master a particular account can not be opened.
SAP delivers some standard field status group for different type of general ledger account which can be used by any one. If you want to have a new field status group then that need to be created by copying an existing field status group.
If you post a document to a sub ledger account then the field status assigned to the reconciliation account in your general ledger account master is used.
Posting key in SAP also manage certain field status for document entry. Since sub ledger do not have a field status group, differentiation in the sub ledger posting is mainly done through different posting keys. Therefore In transaction code OB41 you will find lots of posting key for sub ledger posting but only two posting key for general ledger posting i.e. 40 for debit entry and 50 for credit entry as for each general ledger account master we assign a field status in company code segment.