A: Pattern/Track Write/Play mode selection
Instead of two big rotary switches on the 303, we have a display and buttons to switch between the 4 modes. Pressing a button once will put you into PLAY mode, pressing it again will put you into WRITE mode for the corresponding mode (TRACK/PATTERN). The LED above the button will stay ON when in PLAY mode and flash when in WRITE mode. The display will show the numbers 1-6 for TRACK mode banks and ABCD for PATTERN mode banks.
B: MIDI Connection
The pixie processor supports MIDI SYNC only at this time, to activate midi sync, you press the FUNCTION and CLEAR buttons on the keyboard at the same time (this will trigger the settings mode) and the sync button is toggled with the time mode button (the LED will be on or off depending on the mode). The sequencer can then be started from a midi sequencer that sends a start/stop command and the internal tempo is disabled. Pressing the function button will exit out of the settings mode
C: Analog sync functions
The DINSYNC functionality is unchanged from the 303 but the routing is expanded to make it more flexible in a modular setting. The two sync inputs (RUN and CLOCK) are normalled to the internal clock so without anything plugged in, the device runs as SYNC master, just like the 303. Plugging anything into those ports slaves the device to incoming sync (this MUST be DINSYNC/24PPQN positive clock for the clock and a RUN HIGH/OFF LOW run signal). Since there are two inputs instead of one cable on the 303, those two signals do not have to be both used at the same time so you could slave the clock and still use the panel start/stop or vice versa. There is also a switch on the back of the device that converts those two ports to sync OUTPUTs that send clock and start info (just like plugging in the DIN cable halfway on a 303)
D: Adjustable slide time
The slide circuit has 4 different slide times available. OFF is the stock TB-303 slide time and then the LEDs cycle between the 3 others which get progressively longer
Beyond that, please refer to the TB-303 instruction for sequencer operations. Great care was taken to make sure that old NEC part behavior is accurate and if you've used a TB before, it will be very familiar, very quickly.
The Roland TB-303 Manual can be found here