Simple.
https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_71/db2/rbafzbackup.htm

OVERRIDING SYSTEM VALUE or OVERRIDING USER VALUE
Specifies whether system generated values or user-specified values for a
ROWID, identity, or row change timestamp column are used. If OVERRIDING
SYSTEM VALUE is specified, the implicit or explicit list of columns for
the INSERT statement must contain a ROWID, identity, or row change
timestamp column defined as GENERATED ALWAYS. If OVERRIDING USER VALUE is
specified, the implicit or explicit list of columns for the INSERT
statement must contain a column defined as either GENERATED ALWAYS or
GENERATED BY DEFAULT.
OVERRIDING SYSTEM VALUE
Specifies that the value specified in the VALUES clause or produced by a
fullselect for a ROWID, identity, or row change timestamp column that is
defined as GENERATED ALWAYS is used. A system-generated value is not
inserted.
If a value for a row-begin, row-end, transaction-start-ID, or generated
expression column is provided, it must be DEFAULT.
OVERRIDING USER VALUE
Specifies that the value specified in the VALUES clause or produced by a
fullselect for a column that is defined as either GENERATED ALWAYS or
GENERATED BY DEFAULT is ignored. Instead, a system-generated value is
inserted, overriding the user-specified value.
If neither OVERRIDING SYSTEM VALUE nor OVERRIDING USER VALUE is specified:
A value other than DEFAULT cannot be specified for a ROWID, identity, row
change timestamp, row-begin, row-end, transaction-start-ID, or generated
expression column that is defined as GENERATED ALWAYS.
A value can be specified for a ROWID, identity, or row change timestamp
column that is defined as GENERATED BY DEFAULT. If a value is specified
that value is assigned to the column. However, a value can be inserted
into a ROWID column defined BY DEFAULT only if the specified value is a
valid row ID value that was previously generated by DB2® for z/OS® or DB2
for i. When a value is inserted into an identity or row change timestamp
column defined BY DEFAULT, the database manager does not verify that the
specified value is a unique value for the column unless the identity or
row change timestamp column is the sole key in a unique constraint or
unique index. Without a unique constraint or unique index, the database
manager can guarantee unique values only among the set of system-generated
values as long as NO CYCLE is in effect.
If a value is not specified the database manager generates a new value.


Rob Berendt

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