The “MiddlePane” allows you to add a “panel” on your layout between the upper detail panel and the tabs at the bottom.

Adding a Middle Pane to ACT

Dragging any of the tabs from the Tab control to this panel will move that Tab there temporarily, allowing you to view two tabs at the same time. The easy drag and drop interface means you can switch Tabs on the fly.

Key Features

    Layout based control – Choose which Layouts include the middle pane and which don’t
    Configure the prompt text for the control in design view
    Integrated Drag and drop functionality at runtime

Source: Developer Community, Allen Duet