Any open tab is shown on the project tab list. Clicking the '' button from the tab opener opens an empty tab, whereas clicking the '' button allows to open a panel (or App panel) from a currently open project. For the panel selector features, see here.
Dragging out a project tab out of the tab list creates a new ETS instance with the previously opened panel in.
Dragging a project tab and dropping it to another tab (in the middle gray area of the target tab) modifies the target tab by merging the panel into it.
Dragging a project tab and dropping it to another tab (in the left or right gray area of the target tab) modifies the order of the open tabs.
A tab may also be closed when hovering it and then clicking the middle button of the mouse (or the mouse wheel).