ISX04 - What is the purpose?
My understanding is that ISX03 value will override the standard version of the code lists. Meaning that ALL codes defined in that X12 standard version would be valid. And if the industry sub-set does not define the code but X12 standard does define the code then the value would be valid. So I must be missing something related to the purpose of the ISX04. Unless the override is only to be used with a match on the value in cols 7 through 12 on GS08 with the value in GS08 and if no match then the version override is ignored?
Section 5 of the document "The ISX Segment in Operation" at https://isx.x12.org addresses this RFI.
The purpose of ISX04 is to indicate that only the codes listed within a referenced implementation guide are usable. For example, data element 128, Reference Identification Qualifier has over 1,900 code values. Implementation guides usually don't allow the entire list and publish a subset. For this purpose, that subset is identified by content in ISX04. If a code not explicitly listed in the implementation is sent, that is an error condition when ISX04 is used.