UNB

Interchange Header. The very first segment of any EDIFACT interchange: who's speaking, to whom, when, under which syntax.

Definition

The UNB (Interchange Header) is the mandatory service segment opening every EDIFACT interchange. Its structure and semantics are defined by ISO 9735-1:2002 ("Electronic data interchange for administration, commerce and transport (EDIFACT) — Application level syntax rules — Part 1"), section §C.2. It carries:

1. S001 — Syntax identifier: syntax tag (UNOA, UNOB… UNOC) and its version number (1, 2, 3, 4).
2. S002 — Interchange sender: sender identifier, its qualification code (for instance 14 for GS1 GLN, ZZZ for mutually defined) and an optional routing address.
3. S003 — Interchange recipient: symmetric to the sender.
4. S004 — Preparation date and time: YYMMDD and HHMM of generation.
5. 0020 — Interchange control reference: unique interchange number, which must reappear in the UNZ trailer for validation.
6. S005 — Recipient reference / password, 0026 — Application reference, 0029 — Processing priority, 0031 — Acknowledgement request, 0032 — Communications agreement ID, 0035 — Test indicator (all optional).

Origin

UNB is defined by ISO 9735, published in 1988 under the joint impetus of UN/ECE and ISO. The spec is maintained in the ISO 9735:2002 revision (10 parts) — the version active in 2026. Together with UNZ, UNB makes up the service pair that delimits an interchange; one level deeper, every message is itself delimited by a UNH/UNT pair. Because ISO 9735 is paywalled, ediverse.io cites its sections by number rather than reproducing content.

Example in context

A minimal ORDERS interchange begins as follows:

Reading: UNOC syntax v3, sender GLN 3012345000003 qualified 14 (GS1), recipient GLN 5412345000017 qualified 14, generated on 13 May 2026 at 10:42, interchange control reference IC-2026-0001. The final UNZ must echo the same reference to close the interchange.

Related terms

• Segment — the general concept of which UNB is a special case.
• BGM — the message-level opening segment (UNB is at interchange level).
• NAD — another segment defined in directory D.96A.
• Qualifier — the concept behind every code UNB carries.