The preview window renders a composition in real-time. The checkerboard background in this example is manually set in the document inspector. It will be neglected by just:play and just:live.
1 Send Trigger (lower left): Continues the composition if a stop frame has been reached.
2 State (lower middle): Shows the current status of the preview.
3 Reload (lower right, Shortcut: "R"): Rewinds the preview.