HL7 ADD Addendum
HL7 field reference ADD fields from HL7 v2.5.1 Hide fields
ADD is the HL7 Addendum segment. The ADD segment is used to define the continuation of the prior segment in a continuation message.
Addenda and comment-like segments are best used as supporting text, not as a hiding place for data that a receiver is expected to compute from.
The imported message structures do not list a common message for it in v2.5.1, so I would treat it as profile-driven and avoid populating fields without a clear receiving-system rule.
When you are testing this segment, HL7 Soup Web is handy for checking how the fields split, while Integration Soup is the better place to make routing, validation, and transformation decisions obvious.
So this page stays intentionally practical: what each field is for, what shape the generated v2.5.1 data gives it, and where I would be careful before mapping it.
ADD is not an ordinary note segment. It is part of the continuation mechanism for a segment that was split across messages. When ADD appears after the segment being continued, it can act as a marker with no fields. In a continuation message, the following fields represent the remaining fields from the prior segment.
Only use ADD when both systems have explicitly implemented continuation handling. Most everyday interfaces are better off avoiding it and using normal segment repetitions, NTE comments, or document-style segments where those are appropriate. A receiver that does not understand continuation will not magically stitch the message back together.