I've been reading up on reverse proxying and am wondering when `proxy_set_header Host $host` is appropriate over `proxy_set_header Host $proxy_host`. I did some research and in [this article](https://www.digitalocean.com/community/tutorials/understanding-nginx-http-proxying-load-balancing-buffering-and-caching) it says that in most cases we set Host to $host. Then why does nginx default to $proxy\_host? To help me understand more concretely, will the reverse proxy configuration [here](https://whatididtodowhatidid.wordpress.com/2014/03/14/subdomains-for-ports-on-same-ubuntu-server-with-nginx-reverse-proxy/) (bottom of article) still work if we use $proxy\_host instead? 

Thanks