I've been looking for documentation on the new layer composite spaces in 2.10 and, so far, have not been able to find much. As a result, I started some of my own tests.

Here's what I've uncovered:

Blend Space: according to the GIMP release notes, this switches the mode of layer blending from the old perceptual method and the new linear method. I'm not sure what the Auto setting defaults to; I assume New, but I guess it may also depend on whether a pre-2.10 XCF file is loaded or not(?)

Composite space: I believe this is essentially the same as Blend Space, except this refers to the mode of the layer only rather than the *interaction* between two layers.

Composite modes:

Union: Processes the composite effect and displays all layers. Where there is functionally no compositing happening (ie, in places where there is only alpha), the lowest layer is displayed.

Clip to backdrop: Processes the composite effect and displays all layers. Where there is no compositing happening (ie, in the alpha), only alpha is displayed.

Clip to layer: Processes the composite effect and displays only [this] affected layer (the layer set to this composite mode). Anything not on this layer is treated as alpha.

Intersection: Processes the composite effect and displays only the parts of the affected image that intersect with [this] layer (the layer set to this composite mode). Anything that does not intersect with this layer is treated as alpha.

Attached is a graphical demo of the composite modes. The blend and composite spaces weren't really worth demo'ing, as the differences were very subtle (which, I think, is the intent; the old perceptual method is, from what I've read, only around to ensure that old XCFs display unchanged).