Segment and field index

Use this table to locate a segment in a complete example. Field names and relationships come from public cross-references. Use the X279A1 implementation guide for complete grammar and situational rules.

Segment index

Segment	Purpose	Important fields in this guide	Example pages	Primary reference
ISA	Opens an interchange and establishes its control values and fixed-position delimiters.	ISA13 interchange control, fixed separator positions.	Single, Multiple	STEDI-ISA
IEA	Closes an interchange and declares its group count and matching control.	IEA01 group count, IEA02 control value.	Single, Multiple	STEDI-IEA
GS	Opens a functional group inside an interchange.	GS06 group control value, functional-group identity.	Single, Medicare secondary	STEDI-GS
GE	Closes a functional group and declares its transaction count.	GE01 transaction count, GE02 matching control value.	Single, Multiple	STEDI-GE
ST	Opens a transaction set.	ST01=271, ST02 transaction control, ST03=005010X279A1.	Single, Medicare Advantage	STEDI-ST
SE	Closes a transaction set and declares its segment count.	SE01 count from ST through SE, SE02 matching control.	Single, Multiple	STEDI-SE
BHT	Supplies beginning-of-hierarchical-transaction context.	Transaction purpose, reference, date, and time values.	Single, Medicare secondary	STEDI-BHT
HL	Establishes a hierarchical level and optional parent relationship.	HL01 level ID, HL02 parent ID, HL03 level code, HL04 child indicator.	Single, Multiple	STEDI-HL
NM1	Identifies the party attached to the current hierarchy.	Entity identifier, entity type, name, and example identifier values.	Single, Medicare Advantage	STEDI-NM1
EB	Reports a qualified eligibility or benefit statement.	EB01 status, EB03 service type, and nearby plan/context values.	Single, All complete examples	STEDI-EB
DTP	Supplies a date or date-period related to the current context.	Date qualifier, format qualifier, and example date value.	Medicare secondary, Medicare Advantage	STEDI-DTP
MSG	Carries explanatory text where the example uses a message value.	Example message text read with its surrounding EB and hierarchy.	Primary/secondary, Medicare Advantage	STEDI-MSG; related case: X12-RFI-2628

Control-pair lookup

Read opening and closing segments together: ISA13 with IEA02, GS06 with GE02, and ST02 with SE02. The complete examples also declare group, transaction, and segment counts. Validate those relationships first, then apply the remaining X279A1 rules.