When and Where Turbine get the request's parameter "screen"?

[kai liu]

hi,
  in Turbine.java,doGet method,I found this line:
  
  data.setScreen(data.getParameters().getString("screen"));

  I'm using TDK example.I output $data.screen in login.vm,it's value is "index"。
  
  I've check the html page produced by turbine, no places exists "screen".

  I'm very confused that where and when turbine get this parameter?

thanks.