If a value is an exceptional value (NULL-value), this specifies in what way and why proper information is missing.

When valued in an instance, this attribute signals the imposition of realm-specific constraints. The value of this attribute identifies the realm in question

When valued in an instance, this attribute signals the imposition of constraints defined in an HL7-specified message type. This might be a common type (also known as CMET in the messaging communication environment), or content included within a wrapper. The value of this attribute provides a unique identifier for the type in question.

When valued in an instance, this attribute signals the imposition of a set of template-defined constraints. The value of this attribute provides a unique identifier for the templates in question