RAMPSXB (RAMPS eXpansion Board) is a shield that attaches to the AUX pins of the common 3D printer control board, RAMPS.
It adds 4 additional stepper motor driver carriers, 4 additional thermistors, 4 additional PWM-controlled MOSFETs, EXP1 and EXP2 for an LCD controller (such as the RepRapDiscount Smart Controller), additional VCC pins, 5V pins, regulated TX/RX for an HC-05 chip, and suppression capacitors on the minimum endstops, similar to the RAMPS 1.4.2.
This allows for a variety of combinations, including 6 completely separate extruders, or a mixing extruder and a few fans, or a mixture of voltages (12V on the main RAMPS, and 24V on the RAMPSXB). It was originally designed to run either the E3D Kraken or Diamond Hotend, with some spare heaters for fans, lights or support extruders.
RAMPSXB consumes the maximum endstops, as well as servo pins 2, 3, and 4 to drive the FETs.
More infomation here: http://reprap.org/wiki/RAMPSXB