An Account group controls the data that needs to be entered at the time of creation of a master record. Account groups exist for the definition of a GL account, Vendor and Customer master. It basically controls the fields which pop up during master data creation in SAP.

Field status groups control the fields which come up when the user does the transactions. There are three options for field selection.  They are:
Display only
Suppressed
Mandatory
So basically you can have any field either for display only or you can totally suppress it or make it mandatory. The field status group is stored in the FI GL Master Record.

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