Variables comes either from one of the following
Parent or Inbound Payload
Form variables
Response Headers
Query params
Response Headers
Response headers is available from the parents call to the child call by usin {{responseHeaders.myHeader}}
Log example. Here we can see available variables like {{responseHeaders.Server}} which would resolve to cloudflare. or {{responseHeaders.X-Frame-Options}} which would resolve to SAMEORIGIN in the example above.