Precisions about $upstream_response_time

[B.R.]

http://nginx.org/en/docs/http/ngx_http_upstream_module.html#var_upstream_response_time

Does this represents the time from the end of the request forward to the
upstream until it starts to answer (kinda TTFB)?
Or does that include the whole response has been received, excluding the
time taken to forward it to the client?
Or maybe something else?

Thanks,
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*B. R.*
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