The values of the OBJECT IDENTIFIER type correspond to the nodes of object identifier tree, the tree set up to allow unambiguous naming of information objects. So if you need to use some of the tree nodes, you can exploit this type.
The type notation is very easy:
FileType ::= OBJECT IDENTIFIER
AttributeType ::= OBJECT IDENTIFIER
But the value notation is rather complicated. It si possible to use both numeric and symbolic notations, e.g.:
telephoneNumber AttributeType ::= {2 5 4 20}
directory OBJECT IDENTIFIER ::= {joint/iso/ccitt ds(5)}
attributeType OBJECT IDENTIFIER ::= {directory attributeTypes(4)}
telephoneNumber OBJECT IDENTIFIER ::= {attributeType 20}
Both given examples describe a telephone number as an attribute defined in object identifier tree.
The only subtyping possibilities are single value and contaned subtype.