Line In: Plug your guitar into here if you're just using it to DI an acoustic or go from you're pedal board to the desk.
From Amp: This is where you'd plug in your amp head. Use one of the speaker out jacks from your amp. Use whichever impedance output the speaker you are going to use will be (When using the From Amp feature you must use the To Speaker jack to attach a speaker or it may damage your amp).
To Speaker: From here plug into your cabinet/speaker making sure it matches the impedance from your head.
XLR Out: This is the balnced output to connect to your mixer or recording console.
Ground Lift: If there is a hum in your signal use this to eliminate it.
Cab/4x12: This switch selects which type of cabinet is emulated by the DI Box. Cab emulates a 2x12 cabinet and 4x12 emulates a 4x12 cabinet.