X12 and eligibility terminology

Use this page as a glossary while reading the structure and complete-example pages. The sources page explains which documents apply to implementation work.

Terms

Term	Definition
270	The X12 health care eligibility benefit inquiry sent before a response.
271	The X12 health care eligibility benefit response discussed in this guide.
AAA	A response-level request-validation or rejection segment; it is distinct from transport, TA1, and 999 outcomes.
API	Application programming interface. An API contract can add requirements beyond the X12 transaction.
Benefit	Eligibility, coverage, or service information returned in a response context.
BHT	Beginning of Hierarchical Transaction segment; it supplies transaction context after the transaction-set header.
Clearinghouse	An intermediary that can transport, validate, translate, or route a transaction.
Companion guide	Route-owner documentation that adds local trading-partner instructions to the implementation standard.
Component separator	The ISA-established character that separates components within a composite element.
Control number	An interchange, group, or transaction identifier paired between an opening segment and its closing segment.
DTP	A date or date-period segment interpreted with the hierarchy and benefit context around it.
EB	The Eligibility or Benefit Information segment. Read it as a qualified statement with its surrounding context.
Element	A positional data value after a segment identifier and element separator.
Envelope	A common name for the X12 header-and-trailer pairs around an interchange, functional group, or transaction set.
Functional group	The GS through GE container inside an interchange.
GE	Functional Group Trailer; it closes a GS group and declares its transaction count and control number.
GS	Functional Group Header; it opens a functional group inside an interchange.
HL	The Hierarchical Level segment that identifies a level and optional parent.
HSD	Health Care Services Delivery segment; it conveys service-delivery quantities or measures when the applicable profile uses it. See the public HSD reference.
Implementation guide / TR3	The licensed normative document that defines the complete transaction grammar and situational rules.
Interchange	The outermost X12 container, from ISA through IEA. Multiple interchanges can appear next to one another in a file.
Intermediary	A clearinghouse or other route participant between a submitter and payer.
IEA	Interchange Control Trailer; it closes an ISA interchange and declares the group count and interchange control number.
ISA	Interchange Control Header; it opens an outer interchange and establishes fixed-position control and delimiter values.
JSON	JavaScript Object Notation; an API can map X12 data into a JSON response shape.
LE	Loop Trailer; it closes the additional-information loop opened by a matching LS.
LS	Loop Header; it starts an additional-information loop that LE closes.
MSG	A Message Text segment used for explanatory text in a profile-scoped context.
NM1	The Individual or Organizational Name segment that identifies a party.
Payer	The organization that administers or pays for coverage.
Repetition separator	The ISA-established character used to separate repeated values where the transaction permits them.
Route	The operational path, connectivity method, intermediary, and payer endpoint used for a transaction.
Segment	A record beginning with an identifier such as ST, HL, or EB and ending at the segment terminator.
SE	Transaction Set Trailer; it closes an ST transaction and declares its segment count and control number.
SOAP	Simple Object Access Protocol, which can carry an X12 message for routes that use it.
ST	Transaction Set Header; it opens one transaction set and identifies its type and control number.
Submitter	The organization or system sending an eligibility inquiry or response through a route.
TA1	An interchange-level acknowledgment outcome, separate from a 271 application response.
Transaction set	The ST through SE container carrying one business transaction.
999	A functional acknowledgment that reports named structural or implementation errors; it is separate from a 271.
2120	The entity-information loop context associated with the current hierarchy; its exact use remains profile-scoped.
RFI	Request for Interpretation; an X12 interpretation of a specifically published question, with scope limited to that question. See the X12 RFI catalog.
X279 / X279A1	The X12 implementation convention for the 270/271 health care eligibility transaction pair used in the public examples.

Program terms

Medicare Secondary Payer (MSP) describes a Medicare coordination concept. The CMS-MSP source explains the program. X279A1 and route-specific guides supply the message rules.

Medicare Advantage describes a Medicare health-plan program context. The CMS-MA source explains the program. Each plan and route can use different payer identifiers and benefit codes.