If an HL7 message contains customized data that cannot be included in any segment that is defined for its message type, you can create a custom segment to transmit this data.
By convention, all custom segments begin with the letter Z. For example, a ZPD segment could be created to contain customized patient demographics information. These custom segments are known as Z-segments.
Applications that process HL7 messages are usually configured to ignore HL7 Z-segments that they do not know how to handle.
Note that some vendors use HL7 Z-segments to transmit information that could also be stored in a standard HL7 segment. This can make message processing more difficult
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