There are a number of advanced HTTP options provided for rarely used but quite useful use cases. For most people you should not need this. The configuration for these settings can be found on the HTTP action link from the Proxy Port page.
Forwarded for Header
A forwarded for header allows for upstream proxies (such as Cloudflare, Incapsula etc) to send along the IP address of connecting clients to the next service in line. In this case this option is provided to allow for a situation where we are used in the middle.
Note: this does not exempt the connecting host from any DDoS protection rules and is generally not a recommended configuration.
An option is provided that allows you to specify a custom HTTP host to be sent to the backend server different to what the client provides. This custom hostname can also include the $host variable. This configuration is especially useful for white-labeling software and allowing for dynamic configurations of domains that the backend software may not easily provide.
$host - This is the default configuration. There is no need to change or enable the setting for this. $host.domain.com - the connecting domain name would be provided as a subdomain to your supplied domain name suffix. domain.com - "domain.com" will always be sent to the backend server regardless of the host header received. _ - A single underscore denotes a special mode where no HTTP header will be sent.