You can define a new type (like MDX) that a Swift/Mac OS Document app can do things with. These UTIs can be declared to conform to other UTIs, such as
public.plain-text to allow the clipboard, NSDocument, etc to handle them.
new in 2020, doesn't replace storyboards for launch screens.
"Build Document-Based apps in SwiftUI" -- WWDC 2020
Physical vs Functional UTI tree is the "functional" representation (plain text, spreadsheet, etc) vs the "on-disk" format.
Access a low-level (libSystem) interprocess communication mechanism that is based on serialized property lists.