Panes can be moved and sized in many ways (e.g. docked, tabbed, hidden, floated, resized, spread across one or more monitors etc.). This helps to group window types and organize your workspace for ease of workflow and also helps maximize valuable screen space when many windows are open.
Some panes contain sub-panes which must remain within the parent pane (e.g. Photos within the Photo Windows pane, 3D Viewers within the 3D View Windows pane). Subpanes act just like regular panes and can be docked, tabbed etc. relative to each other but these ‘sub-panes’ must always remain within their parent pane.