On the server, only one DIT content rule can be defined for an entry in the structural object class.

The following formal specification for attribute types is provided in RFC 4512, section 4.1.6.

DITContentRuleDescription = "(" wsp
numericoid                  ; Object identifier of the structural object class the rule applies to
[ sp "NAME" sp qdescrs ]    ; Short name descriptor
[ sp "DESC" sp qdstring ]   ; Description
[ sp "OBSOLETE" ]           ; Specifies if the rule is inactive
[ sp "AUX" sp oids ]        ; List of allowed auxiliary object classes
[ sp "MUST" sp oids ]       ; List of required attributes
[ sp "MAY" sp oids ]        ; List of allowed attributes in the entry
[ sp "NOT" sp oids ]        ; List of prohibited attributes in the entry
extensions wsp ")"          ; Extensions followed by a white space and ")"

The following extensions are specific to the server and aren't defined in RFC 4512.

extensions = / 
"X-ORIGIN" /             ; Specifies where the attribute type is defined
"X-SCHEMA-FILE" /        ; Specifies which schema file contains the definition
"X-READ-ONLY"            ; True or False. Specifies if the file that contains 
                         ;   the schema element is marked as read-only in
                         ;   the server configuration.