Glossary

X12 and gateway terminology, in the form of definitions you can read once and refer back to. If you are new to X12, read this from top to bottom; if you came here from a chapter, jump to the term you need.

X12 envelope

Term	Meaning
Interchange	One ISA/IEA-bounded payload — the transport-level unit. Carries one or more functional groups.
ISA	Interchange Control Header. Opens an interchange; carries sender/receiver qualifiers + IDs, control number, usage indicator (P production / T test).
IEA	Interchange Control Trailer. Closes the ISA.
GS	Functional Group Header. Opens a functional group; identifies what kind of transaction set is inside (HC claim, HS eligibility request, HB eligibility response, HI 278, HP 835, BE 834).
GE	Functional Group Trailer. Closes the GS.
ST	Transaction Set Header. Opens one transaction (one 837, one 835, etc.).
SE	Transaction Set Trailer. Closes the ST and carries the segment count.
Implementation Convention Reference	The version string under HIPAA — e.g. 005010X222A1 for the standard 837P. Sits in GS08.

Loops, segments, elements

Term	Meaning
Segment	One line of an EDI file, terminated by ~. Identified by a 2–3 character ID (NM1, BHT, CLM).
Element	One field inside a segment, separated by *. Position counts from 01. NM103 = the third element of an NM1.
Composite element	An element that contains sub-elements separated by :.
Loop	A logical group of segments (sometimes nested). Identified by 4-digit numbers like 2000A, 2300, 2400.
Hierarchical level (HL)	A loop-organizing segment that establishes a parent/child structure inside an 837. The HL number cascades down (1 → 2 → 3).
Qualifier	A coded value in a segment that says what kind of value follows. NM101 = IL (Insured), 41 (Submitter), 40 (Receiver).

Transactions

Code	Name	Direction (most common)
270	Health Care Eligibility Benefit Inquiry	Outbound
271	Health Care Eligibility Benefit Response	Inbound
276	Health Care Claim Status Inquiry	Outbound
277	Health Care Claim Status Response	Inbound
277CA	Claim Acknowledgment (the BHT02 = 08 flavor of 277)	Inbound
278	Services Review (auth request and response)	Both
820	Premium Payment	(Not used by RCM)
834	Benefit Enrollment & Maintenance	Inbound
835	Health Care Claim Payment / Remittance Advice (ERA)	Inbound
837P	Professional Claim	Outbound
837I	Institutional Claim	Outbound
999	Implementation Acknowledgment (replaces 997)	Inbound
TA1	Interchange-level acknowledgment	Inbound

Acknowledgment hierarchy

Ack	Tells you
TA1	The ISA envelope itself parsed (or didn't). Always optional; some partners skip.
999	The 837 file's functional content was syntactically valid (AK1/AK9) — or it wasn't (IK3/IK4 carry the error detail).
277CA	The payer accepted or rejected each claim individually. The most actionable ack.

Group codes (835 / denials)

Code	Meaning
CO	Contractual Obligation — provider write-off; cannot be billed to member.
PR	Patient Responsibility — bill the member (deductible, coinsurance, copay).
PI	Payer Initiated Reduction — same impact as CO.
OA	Other Adjustment — usually informational.
CR	Correction and Reversal — reverses a prior adjudication.

Trading partner concepts

Term	Meaning
Trading partner	An EDI counterparty (clearinghouse, state Medicaid MMIS, commercial payer). Identified by a name + ISA08 receiver ID.
Submitter	The entity submitting an 837 — usually our tenant. Carries NM1*41 in the 1000A loop.
Receiver	The trading partner. Carries NM1*40 in 1000B.
Companion guide	A per-state or per-payer overlay on the X12 implementation convention (see Companion Guides).
Routing rule	Configuration that picks the correct trading partner for an outbound transaction (see 3.3 — Submission routing rules).

Gateway concepts

Term	Meaning
edi-gateway	The service that builds outbound EDI, parses inbound EDI, manages connections, and exposes the admin API.
edi-app	The web UI for EDI staff (/transactions, /partners, /companion-guides, etc.).
Transaction record	One row in billing.edi_transaction representing a single inbound or outbound X12 transaction.
Batch	A bundle of transactions packaged for transport — typically all of one cron tick's outbound 837s.
Correlation ID	The ID that ties a transaction record to its originating event (claim build, auth request, etc.).
Control number	An ISA / GS / ST sequence number — strictly monotonic per partner per direction. Managed by the control-number-manager service.
Replay	Re-running an inbound or outbound transaction without re-generating the source event (see 8.2 — Replay & retry).

See also

• Segment cheat-sheet — the 30 segments you will see most.
• Acknowledgment codes — TA1/AK/IK/STC/CTX decoded.