In the Apple world, UTI refers to universal type identifiers. It's a hierarchical system that has replaced MIME types and file extensions to describe types of data, whether applications or images, movies or audio. They were first introduced in OS X 10.4 and now are used across both iOS and OS X. UTIs specify what kind of information is being used for common data objects.
Today, I was working on updating an OS X utility of mine, trying to add drag-and-drop to the application icon. To accomplish this, I had to associate document types with my app, so it knew what kinds of items to respond to.
As you've probably already figured out, these document types are defined as UTIs, using Apple's standard reverse domain-naming system.
Immediately I jumped over to Google, where I typed in UTI and public, searching for the kind of UTI that described both folders and generic files. I expected to find something like "public.document".
My top result? "Can you get UTIs from a public toilet" Thank you, Google.
For the curious, the answer I was looking for, by the way, was "public.data". My app is now updated and accepting drag-and-drops on its icon.