notify airlines in advance?

I am flying to Shanghai next week, staying with a friend, then continuing on to Bangkok 48 hours later. According to the above: "For air passengers who are interested to visit Beijing/Shanghai during stopover, they should make sure put forward transit permit requests to the airlines taken in advance, so that the airlines can declare it to the airport customs before landing." I called United Airlines today to find out how I should do this, and they have no idea what I'm talking about. What does this mean?