The Master Brick can be used for two purposes: First of all it has four Bricklet ports and therefore is ideally suited for applications where a many Bricklets are used. These can be directly controlled over the USB interface of the Master Brick. This way USB sensors, like temperature or humidity sensors, or USB actors, like relays, can be created according to your individual needs.
Secondly, the Master Brick can be used for communication purposes. When building a stack the lowermost Master Brick acts as the master of the stack and routes all communication between the boards of the stack and the controlling device. Other Master Bricks in the stack detect that are not the lowermost Master and will only provide their attached Bricklets. This way only one USB connection, the one to the lowermost Master Brick, is necessary.
The USB connection of the Master Brick can be changed with Master Extensions. There are Master Extensions for cable based interfaces (RS485, Ethernet) or wireless interfaces (WIFI). Master Extensions have to be plugged on top of a Master Brick and will be detected as new interfaces. This way Bricks and Bricklets can be directly controlled from devices inside a network (WIFI or Ethernet) or can be interconnected over large distances (RS485).
The maximum stack consists of (bottom to top): 1x Step-Down Power Supply, 1x Master Brick, 8x arbitrary Bricks, 2x Master Extensions. With all Bricks being Master Bricks in a stack up to 9 Master Bricks x 4 Bricklets each = 36 Bricklets can be connected to a single stack.