ant headless will use configurations specified in your bc.conf file. You can change the parameters in there, such as which team is team a and which team is team b.
ant headless -Dbc.game.team-b=testplayer will also include the additional parameters that are passed in via
-D (in this case, it will set team b to testplayer). However, these additional parameters passed in through command line take lower precedence than bc.conf, so if you have team B set in bc.conf, it'll override the command line argument.