In version 1.9.0.1:
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1: Major revision (new UI, lots of new features, conceptual change, etc.)
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9: Minor revision (maybe a change to a search box, 1 feature added, collection of bug fixes)
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0: Bug fix release
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1: Build number (if used)—that’s why you see the .NET framework using something like 2.0.4.2709
You won’t find a lot of apps going down to four levels, 3 is usually sufficient.