# iPhone Duo developer glossary

> Definitions of the iPhone Duo developer vocabulary.

Source: https://iphoneduosupport.com/glossary/


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## App Resizability

The Xcode 27.1 app modernization skill, previously named Modernize your UIKit app, now extended to cover SwiftUI and iPhone Duo. It automates part of the mechanical migration work such as finding fixed sizes and deprecated screen references.

## Arrangement

A layout container that places a primary and a secondary view across iPhone Duo's poses, using size classes, the view's aspect ratio and active division regions as input. Available as ArrangementView in SwiftUI and UIArrangementViewController in UIKit, with split and overlay styles.

## Axis behavior

A property controlling whether a bar item orients vertically when placed in a vertical bar, or stays horizontal regardless. Set with .axisBehavior(.verticalPreferred) or .horizontalOnly in SwiftUI and item.axisBehavior in UIKit.

## Device Hub

The Xcode 27.1 simulator interface for iPhone Duo, with on-screen control buttons to open, close, rotate and fold the device so a layout can be checked in every pose.

## Division region

A reserved region that divides the iPhone Duo inner display into separate usable areas. The hinge is the example: it is active only while the device is folded and has zero width when the device is flat.

## Hinge angle

The physical angle of the iPhone Duo hinge, reported through onHingeChange in SwiftUI and UIHingeInteraction in UIKit, with states closed, partially open and fully open. Apple's guidance is to use it for interactions and effects only, never for layout decisions.

## Multiple scenes

Support for more than one instance of an app at once, declared with UIApplicationSupportsMultipleScenes. iPhone Duo is the first iPhone to support this. New windows can only be created on the inner display; the outer display is reserved.

## Occlusion region

A reserved region that covers part of the display rather than dividing it. On iPhone Duo the under-display FaceTime camera on the inner display is an occlusion region. Background content may pass beneath one; interactive content should not.

## Pose

A physical configuration of iPhone Duo — closed, partially folded like a book, propped on a table, or fully open flat — each combined with an orientation. Xcode's Device Hub simulates poses with on-screen controls to open, close, rotate and fold.

## Reserved region

An area of the screen that system hardware or UI claims, reported by ReservedRegion in SwiftUI and UIViewReservedRegion in UIKit from iOS 27.1. Regions come in two kinds — division and occlusion — and each has an active and an inactive state.

## Safe area

The region of a view not covered by system UI. On iPhone Duo safe areas and layout margins are asymmetric, so each side must be handled independently — code that doubles one inset to account for the opposite side is wrong on this device.

## Scene accessory

Supplementary UI shown on one iPhone Duo display while the app's main UI runs on the other — a teleprompter or subject preview on the outer display, for example. The camera capture accessory is available only when the app is full screen on the inner display with an active camera session.

## Size class

A coarse description of available layout space, either compact or regular in each dimension. On iPhone Duo the inner display is regular in both dimensions and leaves room for sidebars, while the outer display is compact-width and regular-height.

## Split View multitasking

Side-by-side app layout on the iPhone Duo inner display. All apps participate whether or not they opt in, so any app can be given an arbitrary fraction of the display at a moment the user chooses.

## Vertical bar

The shared side region on the iPhone Duo inner display in landscape where navigation, toolbar and tab bar controls lay out vertically instead of horizontally, preserving vertical space for content. Apps built against the iOS 27.1 SDK get it with standard containers.

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Published by iPhone Duo Support (https://iphoneduosupport.com). Not affiliated with Apple Inc.