Di2 shifter buttons can be programmed for different functions and shift patterns to fit each rider's needs. The default Di2 shifter button settings are meant to mimic the actuation and feel of mechanical shifting.
Some riders using 1x11 or 1x12-speed setups or those using Di2’s Synchro Shift mode may prefer to keep their shift patterns simple by assigning both right-hand lever buttons to upshift and both left-hand lever buttons to downshift since no front shifting button is needed in these situations.
Additionally, DURA-ACE and ULTEGRA groups feature a third button on each shifter located on the top of each hood. Riders can program these to shift a derailleur, turn compatible bike lights on and off, or control features on other accessories like compatible cycling computers.