Status codes and reason phrases
In the HTTP response that is sent to a client, the status code, which is a three-digit number, is accompanied by a reason phrase (also known as status text) that summarizes the meaning of the code. Along with the HTTP version of the response, these items are placed in the first line of the response, which is therefore known as the status line.
https://www.ibm.com/docs/en/cics-ts/6.1?topic=concepts-status-codes-reason-phrases