Writes all fields of the field group fg (see FIELD-GROUPS)
as one record to a sequential dataset (paging). If a field group
HEADER has been defined, its fields prefix each record to form a
sort key. You can sort this dataset with SORT and process it with
LOOP ... ENDLOOP . After this, EXTRACT cannot
be executed again.
As soon as the first dataset for a field group has been extracted with EXTRACT, the field group cannot be expanded using INSERT. The field group HEADER, in particular, cannot be expanded after the first EXTRACT (regardless of field group).