They are the countries of the world whose central banks buy U.S. dollars collected by their export businesses, by printing their own money with which to do it, and with those dollars turn around and buy U.S. government securities, not because they necessarily consider them good investments, but primarily because the U.S. government securities market is currently the only U.S. dollar repository big enough to take the bid.
In fact, its even more true for Japan because a weaker currency caused by printing money has major economic benefits for Japanese companies whom will now be able export more.