By now you have likely noticed the GEO append switch when adding a new list:
The GEO append works based on one of two things: IP Address or Zip Code. If a lead comes with an IP address, IntuList will append city, state and country to it. But it will only do so if any of those fields are actually missing. So if a lead comes in with IP, city, state, country already populated, no GEO append will even take place.
If, however, an incoming lead does not have an IP address but instead has a zip code, then IntuList will populate city and state based on the zip code (again, only if those are actually missing). Note that zip code append only works for US zip codes.