diff --git a/docs/user-guide/nginx-configuration/log-format.md b/docs/user-guide/nginx-configuration/log-format.md
index e39c5f695e..4a8a457552 100644
--- a/docs/user-guide/nginx-configuration/log-format.md
+++ b/docs/user-guide/nginx-configuration/log-format.md
@@ -29,7 +29,7 @@ log_format upstreaminfo
 | `$upstream_response_length` | the length of the response obtained from the upstream server |
 | `$upstream_response_time` | time spent on receiving the response from the upstream server as seconds with millisecond resolution |
 | `$upstream_status` | status code of the response obtained from the upstream server |
-| `$req_id` | the randomly generated ID of the request  |
+| `$req_id` | value of the `X-Request-ID` HTTP header. If the header is not set, a randomly generated ID. |
 
 Additional available variables: