When a Unicode value is being coerced into a local character set that has no equivalent character, an error is returned.

If unicode_substitution is set for the session, instead of returning the error, unmapped characters are replaced with a designated substitution character. The substitution character is the default subchar value specified in the mapping file. To override this value, provide a character value for the substitution character with this statement.

Set nounicode_substitution in the session reverts to the default behavior.