Michael Reichmann of The Luminous Landscape has written a typically lucid article on soft proofing:
Soft proofing is simply a mechanism that allows you to view on your computer monitor what your print will look like when it is on paper. A specific paper. That paper and ink combination has been defined by the profile that you or someone else has made for your printer / paper and ink combination. When a printer profile is made the colour of the paper is one of the factors that is figured into the profile, because the spectrophotometer is reading the combination of the ink, and the paper that lies beneath it.
So, if you were able to view your image through the printer profile, you would be able to see how that particular combination of ink and paper would reproduce it, taking into account the gamut as well as other characteristics of the inks used.
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