HTTP::header "Host" and HTTP::host: Any difference?

'HTTP::header "Host"' and 'HTTP::host' seem to return the same thing: the HTTP "Host:" header, according to the irule manual pages I can find.

I wonder what will happen if HTTP 1.0 is used, and there is no "Host:" header sent to the server. How do I get the hostname used by the user? Bear in mind that http::uri does not necessarily contain the hostname unless in a forward proxy situation.