X.500
List of X.500 series standards

ITU-T number
ISO/IEC number
Title of Standard
X.500
ISO/IEC 9594-1
The Directory: Overview of concepts, models and services
X.501
ISO/IEC 9594-2
The Directory: Models
X.509
ISO/IEC 9594-8
The Directory: Public-key and attribute certificate frameworks
X.511
ISO/IEC 9594-3
The Directory: Abstract service definition
X.518
ISO/IEC 9594-4
The Directory: Procedures for distributed operation
X.519
ISO/IEC 9594-5
The Directory: Protocol specifications
X.520
ISO/IEC 9594-6
The Directory: Selected attribute types
X.521
ISO/IEC 9594-7
The Directory: Selected object classes
X.525
ISO/IEC 9594-9
The Directory: Replication
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.