> I don't have the specific definition that you do for
> 'external' or 'URL', so i'm kinda lost by what you
> mean by not a valid URL or mirroring external

There is no site *
There is no page containing *
any url containing * is not valid.

By default httrack stays on one site, the one(s) you entered in the url box.
External sites are all others not listed.
start on your.site.org/page it will not try to mirror google.com even if
there's a link on the page. google.com is external to your.site.org