A field group does not reserve storage space for the 
fields, but contains pointers to existing fields.> Internal 
take up memory. Depending on how much memory your system 
has . Once you have declared the possible record types as 
field groups and defined their structure, you can fill the 
extract dataset using the following statements: EXTRACT. 
When the first EXTRACT statement occurs in a program, the 
system creates the extract dataset and adds the first 
extract record to it. In each subsequent EXTRACT statement, 
the new extract record is added to the dataset EXTRACT 
HEADER. When you extract the data, the record is filled 
with the current values of the corresponding fields. As 
soon as the system has processed the first EXTRACT 
statement for a field group , the structure of the 
corresponding extract record in the extract dataset is 
fixed. You can no longer insert new fields into the field 
groups and HEADER. If you try to modify one of the field 
groups afterwards and use it in another EXTRACT statement, 
a runtime error occurs. By processing EXTRACT statements 
several times using different field groups, you fill the 
extract dataset with records of different length and 
structure. Since you can modify field groups dynamically up 
to their first usage in an EXTRACT statement, extract 
datasets provide the advantage that you need not determine 
the structure at the beginning of the program.