X12 Segment Reference

Understanding EDI X12 Structure

Every EDI X12 document follows a layered envelope structure: the ISA/IEA interchange envelope wraps one or more GS/GE functional groups, which in turn wrap individual ST/SE transaction sets. Within each transaction set, segments are arranged according to the implementation guide for that document type. The ISA segment is always exactly 106 characters and defines the delimiters used throughout the file — element separator, sub-element separator, and segment terminator. Understanding this structure is essential for building, debugging, and maintaining EDI maps.

Segments, Elements, and Qualifiers

Each segment begins with a two- or three-character identifier (N1, PO1, REF, DTM) followed by data elements separated by the element delimiter. Many elements use qualifier codes to specify their meaning — for example, N1*ST identifies a “Ship To” party while N1*BT identifies “Bill To.” The REF segment uses reference identification qualifiers like PO (Purchase Order Number) or VN (Vendor Number). This reference tool covers 43 segments with full element definitions, positions, types, lengths, and the most common qualifier codes used across 27 transaction sets.

Building and Maintaining EDI Maps

EDI mapping — translating between X12 segments and your internal data structures — is where most of the complexity lives. Each trading partner may use different optional segments, qualify elements differently, or require custom loops. I build and maintain EDI maps that handle partner-specific variations, validate against implementation guides, and integrate with ERP and warehouse systems. Learn about EDI services, explore integration capabilities, or get in touch to discuss your mapping requirements.