Commonly, scheme, host, and port.
If a proxy receives a host name which is not a fully qualified domain name, it MAY add its domain to the host name it received.
In requests that they forward, transparent proxies MUST NOT rewrite the “abs_path” part of a Request-URI in any way except to replace a null abs_path with “*”, no matter what the proxy does in its internal implementation. This “no rewrite” rule prevents the proxy from changing the meaning of the request when the origin server is improperly using a non-reserved URL character for a reserved purpose. Implementers should be aware that some pre-HTTP/1.1 proxies have been known to rewrite the Request-URI.
By itself, the “abs_path” is a Relative URI.