Documentation of "proxy_set_header Proxy" field

[jjarokergc]

The Proxy Set Header Array in init.pp sets the following field in nginx config file

    proxy_set_header      Proxy "";

What is the intent of setting this field in the proxy_set_header directive?

There is no "Proxy" field documented for HTTP headers. However, there is a return field "PROXY" used by javascript functions, but that does not appear to be an http header. In the context of javascript operating on the client, setting Proxy to null appears to block the client from inferring whether it is communicating with a proxy instead of the actual server.

Is it correct to place "Proxy" field into the proxy_set_header directive and, if so, can someone point me to the documentation.