Explain Field Group (extract dataset)?

An extract dataset consists of a sequence of records. These records may have different structures. All records with the same structure form a record type. You must define each record type of an extract dataset as a field group, using the FIELD-GROUPS statement.
FIELD-GROUPS <fg>.
This statement defines a field group <fg>. A field group combines several fields under one name. For clarity, you should declare your field groups at the end of the declaration part of your program.
A field group does not reserve storage space for the fields, but contains pointers to existing fields. When filling the extract dataset with records, these pointers determine the contents of the stored records.