entity identifier

The specified series, the assigning authority, is defined by components 2 through 4. The assigning authority is of the hierarchic designator data type, but it is defined as three separate components in the EI data type, rather than as a single component as would normally be the case. This is in order to maintain backward compatibility with the EI's use as a component in several existing data fields. Otherwise, the components 2 through 4 are as defined in Section 2.8.18, "HD - hierarchic designator." Hierarchic designators are unique across a given HL7 implementation.