Layers_Extra
These layers contain various bones that don't fit in elsewhere.
The arm and leg sockets are the parents of all arm and leg bones. Unlike the twist bones, moving a socket bone moves the entire underlying hierarchy.
If Ankle IK is turned on, the ankle bones on the Extra layers become the IK effectors instead of the IK foot bones. The feet are posed with the FK foot and toe bones. This mix of IK and FK bones needed also appear on the Extra layers.
The Extra layers also contains bones which determine the location of the toe, ball, and heel. These bones are used for automatically keeping all parts of the foot above ground, especially for characters in high heels. If you don't intend to use the tools that keep the foot above ground, these bones can be ignored.
By default the location of the marker bones is determined by the foot and toe bones. This may be a good approximation to the bottom of the foot for barefoot characters, but not for character in high heels.
Change location of the marker bones in edit mode
Now the markers indicate the location of the foot's toe, ball and heel.