(1) Would it be helpful to mention that in interface terms the addressLine data can be gathered in various ways (e.g., via a textarea element or multiple input elements)? The word "line" might make it sound as if there can be only one of them, whereas in the spec it's a FrozenArray and we might not want implementers to think they can have only one.

(2) In addition to the currently-mentioned constructs ("a street name, a house number, apartment number, a rural delivery route, descriptive instructions, or a post office box number") we might consider mentioning other common premise details like a suite or floor number, a building name, a department name, or a mail stop. Existing standardized address formats (e.g., OASIS xAL) have defined specialized fields for such constructs, but as far as I can see they can be included in addressLine.