For the DFPlayer Mini, what are DFPLAYER_DEVICE_U_DISK and DFPLAYER_DEVICE_AUX modes?

The example code here https://wiki.dfrobot.com/DFPlayer_Mini_SKU_DFR0299 has the following lines:

//----Set device we use SD as default----
//  myDFPlayer.outputDevice(DFPLAYER_DEVICE_U_DISK);
myDFPlayer.outputDevice(DFPLAYER_DEVICE_SD);
//  myDFPlayer.outputDevice(DFPLAYER_DEVICE_AUX);
//  myDFPlayer.outputDevice(DFPLAYER_DEVICE_SLEEP);
//  myDFPlayer.outputDevice(DFPLAYER_DEVICE_FLASH);

I understand that the player can handle SD cards and USB sticks but what are AUX and U_DISK?