# UNA is a courtesy, not a mystery — the EDIFACT envelope for people who learned X12 first > transactions.dev · by visibility.cloud — 2026-07-31 > Written for the supply-chain ISV / traceability-vendor engineering org — the integration engineer whose first international partner just sent a DESADV. One thesis: **EDIFACT's service segments do the same envelope work X12's do — with defaulted delimiters instead of declared ones, one less nesting level by default, and an escape character X12 never had — and every difference an integrator actually hits fits on this page.** If you learned on ISA and ST, you already understand UNB and UNH; what follows is the delta, not a second education. ## UNA: six characters, all optional Where X12 [declares its delimiters by position in a mandatory header](/blog/byte-4-is-the-law/), EDIFACT ships defaults and lets a file override them. The override is the UNA segment — the string `UNA` followed by exactly six characters, in fixed order: ``` UNA:+.? ' │││││└─ segment terminator (') ││││└── repetition separator (space = unused, pre-D.99 files) │││└─── release (escape) char (?) ││└──── decimal mark (.) │└───── element separator (+) └────── component separator (:) ``` No UNA, and the defaults apply — which are exactly the characters shown. In practice most files either omit UNA entirely or send it carrying the defaults anyway, as a courtesy to naive readers. A correct lexer honors it when present and assumes the defaults when absent; that is the entire delimiter story, and it is one *if* statement where X12's is four positional reads. ## The envelope levels, X12-to-EDIFACT Level by level, over synthetic fragments: ``` interchange ISA … IEA ↔ UNB+UNOA:3+SELLERGLN:14+BUYERGLN:14+260731:0915+101' … UNZ+1+101' group GS … GE ↔ (UNG … UNE — optional, rarely used) message/set ST*856*0001 … SE ↔ UNH+1+DESADV:D:96A:UN:EAN005' … UNT+7+1' ``` What maps directly: UNZ echoes the interchange control reference from UNB and asserts a count, exactly like IEA; UNT asserts the message's segment count (including UNH and UNT) and echoes the message reference from UNH, exactly like SE — so [the six agreement invariants](/blog/the-envelope-must-agree-with-itself/) port with the segment names swapped and one difference in what the count field covers. What does not map: the functional-group level is *optional* in EDIFACT (UNG/UNE exist but most interchanges skip them), so the default nesting is two levels, not three — UNZ's count is then messages, not groups. And the UNH carries what ST never had room for: the full message identity — type `DESADV`, version `D`, release `96A`, controlling agency `UN`, and an association code like `EAN005` naming the EANCOM subset. Where X12 puts the version on the *group* (GS08), EDIFACT puts it on every message, which is operationally kinder: a mixed-version interchange is self-labeling. Two smaller deltas an integrator hits in week one: EDIFACT composites are first-class and everywhere (X12 uses its component separator sparingly; EDIFACT's `:` is structural in most segments), and numeric conventions differ — the UNA even declares the decimal mark, because `1234,56` is a legal way to say what X12 would only ever write as `1234.56`. ## The `?` mechanic X12 doesn't have X12 has no escape character: if your data contains the element separator, you have a problem the standard does not solve for you. EDIFACT solves it — the release character `?` neutralizes the following character, so `O?+M` is the literal string `O+M`, `??` is a literal `?`, and a company named `Smith ? Sons` survives the wire. The lexer consequence: an EDIFACT tokenizer is a two-character lookahead machine, and a byte-exact round-trip must preserve release sequences as sent, not re-derive them — the `emit(parse(x)) === x` law counts `?+` and any semantically equivalent re-escaping as different bytes, because they are. ## Why this rail gets the deep reference There is a licensing asymmetry under this post. X12 sells access to its standard, so our X12 pages are original prose with segment detail linked to [the licensed reference](https://www.stedi.com/edi/x12), never republished. UNECE publishes the complete UN/EDIFACT directories free of charge — which is why [the EDIFACT side of the family reference goes dictionary-deep](/blog/edifact-is-open-we-pinned-it/), with every vendored directory file sha256-pinned under the same journaled-ruling governance as the family's GS1 artifacts. The flagship walkthroughs are live: the [EANCOM DESADV](/blog/eancom-desadv-walkthrough/) — the international ASN, the message the UNH above introduces — and [EDIFACT ORDERS](/blog/edifact-orders-the-open-purchase-order/), its purchase-order counterpart. That DESADV is not a curiosity. GS1's own adoption numbers put EANCOM far ahead of GS1 XML in installed base, and [the DESADV compiles onto the event record](/blog/eancom-desadv-walkthrough/) exactly as the 856 does — CPS hierarchy in place of HL loops, NAD-carried GLNs landing in `sourceList` and `destinationList`, SSCCs into the aggregation tree ([the X12 twin of that story](/blog/the-856-is-already-an-aggregation-tree/)), and the whole output validating against the pinned official EPCIS 2.0 schema. Conformant, and a superset where the message carries more than the standard models — the pairing that holds everywhere in this family. ## The one-page summary Reading an EDIFACT interchange after years of X12: honor UNA or assume its defaults; expect two envelope levels, not three; take the message identity from UNH, not the group; port the count-and-echo invariants as-is; respect `?` in the lexer; and let the decimal mark surprise you only once. The parse, validate, and compile verbs treat both rails as dialects over [one canonical form](/blog/byte-4-is-the-law/) — when your first DESADV arrives, the pipeline that lands it on the record is behind [the access flow](/get-access/), and the survey asks which rail your partners speak precisely so the right reference meets you at the door. --- Access: https://transactions.dev/get-access/ · All posts: https://transactions.dev/blog/ · Machine face: https://transactions.dev/llms.txt