A variable-length item has a 2- or 4-byte
prefix that stores the current length of the item.
The size of the prefix is specified by the second parameter of the
VARCHAR, VARGRAPH, or VARUCS2
keyword, or by the first parameter of the VARYING keyword.
The parameter must be 2 or 4. If you
do not specify the prefix size, a size of 2 is assumed if the specified
length is between 1 and 65535; otherwise, a size of 4 is assumed. You can
specify any prefix size for definitions whose length is
between 1 and 65535. A prefix size of 2 cannot be specified for
definitions whose length is greater than 65535 since 4 bytes are required
to store the length. 
For more information, see Variable-Length Character, Graphic and UCS-2 Formats.