+These can be excluded by specifying @subcmd{DELETE} on the @subcmd{UNSELECTED}
+subcommand. Specifying @subcmd{RETAIN} makes the default explicit.
+
+The @subcmd{UNCOMPRESSED}, @subcmd{COMPRESSED}, and
+@subcmd{ZCOMPRESSED} subcommand determine the system file's
+compression level:
+
+@table @code
+@item UNCOMPRESSED
+Data is not compressed. Each numeric value uses 8 bytes of disk
+space. Each string value uses one byte per column width, rounded up
+to a multiple of 8 bytes.
+
+@item COMPRESSED
+Data is compressed with a simple algorithm. Each integer numeric
+value between @minus{}99 and 151, inclusive, or system missing value
+uses one byte of disk space. Each 8-byte segment of a string that
+consists only of spaces uses 1 byte. Any other numeric value or
+8-byte string segment uses 9 bytes of disk space.
+
+@item ZCOMPRESSED
+Data is compressed with the ``deflate'' compression algorithm
+specified in RFC@tie{}1951 (the same algorithm used by
+@command{gzip}). Files written with this compression level cannot be
+read by PSPP 0.8.1 or earlier or by SPSS 20 or earlier.
+@end table