The Functional Acknowledgment. It confirms that a functional group was received and passed (or failed) a syntax check. It does not mean the order was approved or the invoice will be paid — only that the EDI was readable, or where it was not.

What do the AK5 and AK9 codes mean?

AK501 is the verdict for one transaction set: A accepted, E accepted but errors were noted, R rejected (M, W and X are rejections for authentication and encryption failures). AK901 gives the same verdict for the whole functional group, followed by how many transaction sets were included, received and accepted.

What is the difference between a 997 and a 999?

Both acknowledge receipt. The 999 Implementation Acknowledgment also reports errors against an implementation guide, not just the base standard; HIPAA healthcare transactions use the 999, while supply-chain X12 mostly still uses the 997.

EDI 997 Functional Acknowledgment Errors, Decoded

The 997 functional acknowledgment is the transaction set that tells you whether the last thing you sent was accepted. When it says "rejected", the useful information is buried in AK3, AK4, and AK5 — and those are numeric codes with no text explanation.

A 997 reporting a rejection

ST*997*0001~
AK1*PO*1~
AK2*850*0001~
AK3*PO1*12**8~
AK4*4**7*ABC~
AK5*R*5~
AK9*R*1*1*0~
SE*8*0001~

Read from the bottom up and it says: one transaction set received, zero accepted, rejected because of an element error in the twelfth segment.

AK1 and AK2: what is being acknowledged

AK1*PO*1 — acknowledging functional group PO (purchase orders) with group control number 1. That number must match the GS06 of the group you sent.

AK2*850*0001 — specifically the 850 with transaction set control number 0001, matching your ST02.

AK3: which segment went wrong

AK3*PO1*12**8~

AK301 = PO1 — the segment ID at fault.
AK302 = 12 — its position in the transaction set, counting ST as 1.
AK303 — loop identifier, empty here.
AK304 = 8 — the segment syntax error code.

AK304	Meaning
1	Unrecognised segment ID
2	Unexpected segment
3	Mandatory segment missing
4	Loop occurs over maximum times
5	Segment exceeds maximum use
6	Segment not in defined transaction set
7	Segment not in proper sequence
8	Segment has data element errors

Code 8 is a pointer, not a diagnosis — it means "look at the AK4 that follows".

AK4: which element, and why

AK4*4**7*ABC~

AK401 = 4 — element position within the segment. So PO104, the unit price.
AK402 — X12 data element reference number, often omitted.
AK403 = 7 — the data element syntax error code.
AK404 = ABC — a copy of the offending data.

AK403	Meaning
1	Mandatory data element missing
2	Conditional required data element missing
3	Too many data elements
4	Data element too short
5	Data element too long
6	Invalid character in data element
7	Invalid code value
8	Invalid date
9	Invalid time
10	Exclusion condition violated

So the full reading: the twelfth segment is a PO1; its fourth element contained ABC, which is not a valid code value. A unit price of ABC — almost always a mapper writing a string where a number belongs.

AK5 and AK9: the verdicts

AK5 is the verdict on one transaction set. AK501 carries the code:

AK501	Meaning
`A`	Accepted
`E`	Accepted, but errors were noted
`R`	Rejected
`M`	Rejected — message authentication code failed
`W`	Rejected — assurance failed on validity tests
`X`	Rejected — content could not be analysed after decryption

AK9 is the verdict on the whole functional group: AK9*R*1*1*0 means rejected, 1 transaction set included, 1 received, 0 accepted.

The distinction that catches people: AK501 = E means the partner accepted your document despite noting errors. You do not need to resend. Teams that treat any AK3 presence as failure end up sending duplicates.

Tracing it back

The whole exercise is turning AK302 = 12 into "the segment on line 12 of the file I sent". Counting segments by hand in a thousand-line interchange is where the time goes — a tool that shows segment positions directly turns a ten-minute count into a jump.