Document types on OS X are defined by what Apple calls Uniform Type Identifiers, or UTIs. HTML files, for example, have a UTI of public.html; Microsoft Word documents are described by the UTI com.microsoft.word.doc. Apple provides many UTIs with OS X, but developers are free to define their own, as long as the UTI is unique to that document type.
duti's name means "default for UTI."
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