Transaction layer could be implied in the context of HTTP response. Thus stating "HTTP Response" should suffice to express the intent without getting too verbose in the text.

I propose a similar edit in the following section:

5.2.2 http://hl7.org/fhir/us/davinci-cdex/2022Sep/sending-attachments.html#technical-workflow

3. Payer responds with an HTTP transaction layer response which either accepts or rejects the transaction