Difference between Byte and Char in Oracle
If you define the field as VARCHAR2(11 BYTE), Oracle will allocate 11 bytes for storage, but you may not actually be able to store 11 characters in the field, because some of them take more than one byte to store, e.g. non-English characters. By defining the field as VARCHAR2(11 CHAR) you tell Oracle to allocate enough space to store 11 characters, no matter how many bytes it takes to store each one. I believe that in Oracle 10g, 3 bytes per character were used.