Working with Toolboxes
Toolboxes are hierarchical; that is, there are "parent-child" relationships between toolboxes. The "parent" toolbox displays one tool from each of its "child" toolboxes.
For example, the Main toolbox is the parent of several "child" toolboxes, including the Fence toolbox. The Fence toolbox's most recently selected tool — Place Fence by default — displays as the second tool in the Main toolbox.
You can select any tool that is a member of a "child" toolbox from that toolbox's "parent" without floating the "child" toolbox. This aspect of toolboxes helps you minimize the screen space they occupy.