| 1 | A | B | C |
| --- | --- | --- | --- |
| 2 | X.500 - List of X.500 series standards |  |  |
| 3 | ITU-T number | ISO/IEC number | Title of Standard |
| 4 | X.500 | ISO/IEC 9594-1 | The Directory: Overview of concepts, models and services |
| 5 | X.501 | ISO/IEC 9594-2 | The Directory: Models |
| 6 | X.509 | ISO/IEC 9594-8 | The Directory: Public-key and attribute certificate frameworks |
| 7 | X.511 | ISO/IEC 9594-3 | The Directory: Abstract service definition |
| 8 | X.518 | ISO/IEC 9594-4 | The Directory: Procedures for distributed operation |
| 9 | X.519 | ISO/IEC 9594-5 | The Directory: Protocol specifications |
| 10 | X.520 | ISO/IEC 9594-6 | The Directory: Selected attribute types |
| 11 | X.521 | ISO/IEC 9594-7 | The Directory: Selected object classes |
| 12 | X.525 | ISO/IEC 9594-9 | The Directory: Replication |
| 13 | X.530 | ISO/IEC 9594-10 | The Directory: Use of systems management for administration of the Directory |

Question: What ITU-T Number is used for Attribute types and object classes? 
Answer: X.520 and X.521 is for attributes and object classes.