The fourth circle in the iOS battery column is a reserved position, which will be displayed if a new device is added, such as the back-clip battery from Apple's official website, Apple watch, Airpods or other beats headphones.

iOS is a mobile operating system developed by Apple. Apple first announced this system at the Macworld conference on January 9, 2007. It was originally designed for iPhone, and was later applied to iPod touch and iPad. iOS, like Apple's macOS operating system, is a Unix-like commercial operating system.

Originally, this system was called iPhone OS. Because iPad, iPhone, and iPod touch all use iPhone OS, it was announced at WWDC in 2010 that it was renamed to iOS (iOS is a registered trademark of Cisco Network Equipment Operating System of the United States, Apple The name change has been authorized by Cisco Corporation).

Apple iOS Siri:

Siri lets you use your voice to send messages, schedule meetings, check the latest scores, and more. Just say what you want to do and Siri will do it for you. Siri can understand what you say, know your thoughts, and even respond. Siri in iOS has a new look, new voice, and new features.

The interface has been redesigned to fade into view and appear on top of any screen. Siri answers questions faster and can query more information sources, such as Wikipedia. It can take on more tasks like returning calls, playing voicemails, adjusting screen brightness, and more.