The definition of page-spread-center is:

The rendition:page-spread-center property specifies the forced placement of a Content Document in a Synthetic Spread.

But this doesn't really tell you anything, not to mention it would be impossible to force a centered document into a two-page spread. Then after the property definitions comes this:

The rendition:page-spread-center property indicates to override the synthetic spread mode and render a single viewport positioned at the center of the screen.

This is much clearer about what to render, but isn't this completely contradictory to the definition? Is there a spread if it overrides spread mode? How are we forcing the document into a spread at the same time we're overriding spreads?

If there's only a single centered viewport, that's not the same as merging the two halves of a spread to make one dual-page spread.

I'd hazard a guess the property would make more sense if it were called "rendition:page-center", since it seems to have little to do with spreads other than to override them.

I'm not sure how best to make sense of the current contradictions, though. All I can think of off the top of my head is to maybe make a separate subsection to explain "page-spread-center" so it's less confusing with the -left and -right properties. Dropping the current definition about forcing a document into a synthetic spread would also help.