A cel (from celluloid) is one image in a specific frame and layer, at a specific xy-coordinate in the canvas.
The difference between a frame and a cel, is that a frame is the set of cels for all layers in a specific time:
You can use the Move tool to drag-and-drop the current cel in the canvas. Or you can use the Timeline to move the cel to another layer/frame position.
On RGB images, each cel has its own opacity level. You can change a cel's opacity by right-clicking it and selecting Properties (or Frame > Cel Properties), and then adjusting the Opacity slider.
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