Last December, Apple gave its developers a heads-up about a new rule requiring that all new apps and app updates for iPhone, including universal apps, be built with the iOS 12 SDK and support the new iPhone XS Max form factor.

Today, the company warned developers that all new and updated iPhone software submitted to App Store on and after March 27, 2019 must be built at least with the iOS 12.1 SDK and support the new 2,688×1,242 pixel resolutions on iPhone XS Max.

This means that any apps built against the latest SDK will run in fullscreen display mode on the latest devices, taking full advantage of all the pixels.

Screenshots for these devices will also be required.

Furthermore, all new and updated iPad or universal apps must be optimized for the native display resolution of the latest 12.9-inch iPad Pro model, which has the highest resolution of any iOS device at 2,732×2,048 pixels.

Apple explains that apps which take full advantage of adaptive UI layouts and safe area insets should look great on iPhone XS Max with minimal effort on the developer’s part.The new requirements are meant to prevent the proliferation of unoptimized apps that on the Max look blown up with fuzzy text/images and black borders.

On top of that, Apple also requires that all new apps and updates submitted to watchOS App Store on or after March 27 be built with the watchOS 5.1 SDK or later and support the latest Apple Watch Series 4 model.

Thanks to the bigger display with rounded corners and more pixels, Apple Watch Series 4 can display more information, but apps must be optimized to take the full advantage of the increased pixel count.

Last but not least, Apple wrote this about to clarify the changes in memory accounting:

iOS 12 and tvOS 12 require apps to use memory far more efficiently than before. If you have difficulty reducing your app’s memory requirements, contact us to request an entitlement for your app to use iOS 11-style memory accounting.

iOS 12 is now running on more than 80 percent of devices worldwide.