The default will be whatever your NLS_LENGTH_SEMANTICS parameter is set to. By default, that is BYTE to be consistent with older versions of Oracle where there was no option to use character length semantics. If you are defining your own schema and you are using a variable width character set (like AL32UTF8), I’d strongly recommend setting NLS_LENGTH_SEMANTICS to CHAR because you almost always intended to specify lengths in characters not in bytes.