I haven't tested it and would certainly like things such as this to be documented but I'd imagine a simple test should confirm what it does (which I will likely do myself soon).
My undertanding is...
Save As (which was the only option until now) = Creates a new project file of the current project. This leaves the previous saved version intact/as is (unless you specifically open and edit it) and now any changes made to the new Save As will be applied as soon as you click Save.
Save Copy As (new option) = Takes a snapshot of the project as is and sets it aside. You continue working in the original project and any Saves will only apply to the project you are working on. The Save Copy As version remains untouched (obviously).
Simple enough but the implication is that in theory now you can start your project, let's call it Platinum 1 Main, and use that project all the way until you complete the project. As you go you use Save Copy As to create backups at crucial points (Drums Tracked, Guits Tracked, Mix 1, Final Mix, Etc). This way if you main project goes nipples up you go back to your last Save Copy As.
To do this with the old Save As you would have to create TWO Save As copies of the project. The first one to take the snapshot and the second one to continue working with... which eventually would need to be Saved As twice again at each stage. This takes up more disk space (not much but it all adds up) and if you are obsessive about creating backup points your project folder ends up HUGE.
If your main, ongoing project gets corrupted (Platinum 1 Main) you revert to you last Save Copy As project, do a Save As (or Save Copy As) of that version to "Platinum Main 2" and that is now your new main working copy until you finish the project or experience another corruption event.
I know you probably know that already but based on some recent threads I think some people are having a hard time seeing the uses for this. I will definitely be using it.
Cheers.