What is a "copy"?

[Joe_Groff]

@vanvoorden

But then what does that mean? Are "identical" and "interchangeable" defined anywhere else across Swift? I don't see much more discussion about what that means or what that implies.

For a Copyable type, copies are literally interchangeable with each other without changing the surface meaning of the program (though obviously can affect the performance characteristics). If you have

foo(a)

then

let b = copy a
foo(b)

will behave exactly the same. The compiler by this principle is free to insert or remove copies to improve performance or maintain language semantics.