From Synfig Studio :: Documentation
The "Placement Widget" (or DockDropArea) is a graphical control element placed in top right corner of each Panels. The "Placement Widget" is composed of four buttons helping to organize the placement of the Panels composing the synfigstudio's Interface.
Those buttons are the places to drop down the panels tabs to arrange them in different configurations : "Up To", "Down To", "Right Side" or "Left Side". Subdivide the panel it belongs to into two panels and the dragged tab would be in the relative place where you drop the tab over those four buttons.
You can take a look to this video showing the use of the "Placement Widget".
Documentation writers note
Project used for the screenshot : File:PlacementWidget.sfg