What is the difference between a chest piece and a half-canvas?

I thought a half-canvassed one have a canvas across the chest, meaning if one can feel a third fabric between the chest's outer shell and the lining, then the jacket is at least half-canvassed.

I've just learned that there is something called a chest piece, apparently seperate from the canvas. So what's the difference between a half-canvassed and a fused one with a chest piece? How can I tell one from the other?
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