Difference between request _time vs upstream_response_time

As per my understanding upstream_response_time is subset of
request_time.

request_time : This shows how long Nginx dealt with the request

upstream_response_time: Gives us the time it took our upstream server

(in this case Apache/mod_wsgi) to respond

request_time = time nginx got the request to the time it sends the
reponse to client(browser).

request_time = variable component + upstream_response_time

Can anybody explain me what exactly the “variable component” is ?

Regards
Kamalakar

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