All attributes in a particular entry must be of different attribute types. Each attribute type is assigned a unique object identifier value. The directory standard assigns object identifiers for several commonly used attribute types, including surname, country name, telephone number, and presentation address. Other international standards may define additional attribute types. For example, the X.400 Message Handling standard defines mail-specific attributes like O/R address. It is expected that various national and private organizations will also define attribute types of their own. The CDS attributes (defined in the xdscds.h header file) and the GDS package attributes (defined in the xdsgds.h header file) are examples of additional attribute definitions.