What is the Host Header?
Many web servers are configured to serve content for multiple sites from the same IP address. The host header tells the web server which site to load content from. It's sent as part of the HTTP request, for example:
GET / HTTP/1.1
Accept-Encoding: gzip, deflate
What Does StackPath do with it?
Setting a domain in the Host Header option will cause the CDN to use that domain in the host header for every request proxied to your origin, and will specify what website is requested from your origin server.
This is important to set correctly, or you may experience errors such as redirect loops, or proxied connection failures because the origin server either may return a response to redirect the request to a separate domain, or may not return a response at all, while the CDN will continue to proxy requests with an incorrect host header.
What is the dynamic host header setting?
The Dynamic option in the drop-down allows the CDN to pass-through the incoming host header from end-user requests to use the same header for requests toward your origin.
This can be useful if you have multiple subdomains for the site that are intended to load different pages, like language translations. Or, if you have multiple websites hosted on the same server, you can use a single StackPath Site to proxy and cache all of them.