In this mode character rig is represented by a set of cuboids known as Box Controllers. These box controllers can be selected and edited in the same manner as Point Controllers or any other parts of the rig.
A character in Box Controller mode
This mode is rarely used for setting the pose of a character, but can be very convenient when you need to adjust small details of a character model such as fingers or parts of the clothes.
An example of using Box Controller mode to position character fingers
Note: The example above utilizes Tail Mode to make adjusting fingers more convenient.