Usb selector setting for pcdj dex 3

can anyone tell me which is the correct setting on back of denon MC6000MK2 to use, there are three settings, LN 3/4 THRU TO PC USB AUDIO OUT CH 3/4 REC OUT/MIC I use pcdj Dex 3 from laptop on chanel 1and 2 and an ipad on ch 3 and cd player on ch 4 which one should i use or does it matter thanks