To me, it's clear that they were thinking of the top of a plain piece of furniture - a desk - which we had been using since offices were invented! The idea was that you could spread all of your work out on the top of a desk and then click on the document you wanted to work on (and bring it forward - also a Xerox term). The metaphore was the top of a typical desk - with piles of files, documents and books at hand to assist in completing your work.
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