What is it for?
The Allocated length is only used when creating files on IBM i. It defines the space to be reserved for variable length columns in each row. Column values with lengths less than or equal to the allocated value are stored in the fixed-length portion of the row. Column values with lengths greater than the allocated value are stored in the variable-length portion of the row and require additional input/output operations to retrieve.
The allocated length cannot exceed the 1.2.4 Field Length.
Allocated length is only available with these field types: String, NVarChar, VarBinary, CLOB & BLOB.
If the field has keyboard shift E, O, or J, the allocated field length must be 0, or greater than 4.
If the field has the attribute SUNI, the allocated field length is specified in characters, not bytes.
If you enter an allocated length of 32000, and the file is created via DDS, VARLEN(32000) will be specified for the field definition.
If the file is created via SQL's CREATE TABLE statement, ALLOCATE(32000) will be specified for the column definition.
For further details, please refer to the IBM DDS Reference and SQL Reference manuals.