The AK4 data segment is created in the functional acknowledgment
when an error is found in a data element in the subject
document. If the functional acknowledgment is enabled, the
Functional Acknowledgment (997)
is created automatically. This data segment is then created in
the transaction set for each data element error found in the subject
document. The error is recorded in Data Element Syntax Error Code
(723) data
element. This data segment is not present if no error is
found.