is often the focus of interaction with their device, and the interface entails components of both hardware and software. User input allows for the users to manipulate a system, and device's output allows the system to indicate the effects of the users' manipulation. Mobile UI design constraints include limited attention and form factors, such as a mobile device's screen size for a user's hand. Mobile UI contexts signal cues from user activity, such as location and scheduling that can be shown from user interactions within a mobile application. Overall, mobile UI design's goal is primarily for an understandable, user-friendly interface. Mobile UIs, or front-ends, rely on mobile back-ends to support access to enterprise systems. The mobile back-end facilitates data routing, security, authentication, authorization, working off-line, and service orchestration. This functionality is supported by a mix of middleware components including mobile app servers, Mobile Backend as a service (MBaaS), and SOA infrastructure. Conversational interfaces display the computer interface and present interactions through text instead of graphic elements. They emulate conversations with real humans. There are two main types of conversational interfaces: voice assistants (like the Amazon Echo) and chatbots. Conversational interfaces are growing particularly practical as users are starting to feel overwhelmed with mobile apps (a term known as "app fatigue"). David Limp, Amazon's senior vice president of devices, says in an interview with Bloomberg, "We believe the next big platform is voice." Distribution The three biggest app stores are Google Play for Android, App Store for iOS, and Microsoft Store for Windows 10, Windows 10 Mobile, and Xbox One. Google Play Google Play (formerly known as the Android Market) is an international online software store developed by Google for Android devices. It opened in October 2008. In July 2013, the number of apps downloaded via the Google Play Store surpassed 50 billion, of the over 1 million apps available. As of September 2016, according to Statista the number of apps available exceeded 2.4 million. Over 80% of apps in the Google Play Store are free to download. The store generated a revenue of 6 billion U.S. dollars in 2015. App Store Apple's App Store for iOS and iPadOS was not the first app distribution service, but it ignited the mobile revolution and was opened on July 10, 2008, and as of September 2016, reported over 140 billion downloads. The original AppStore was first demonstrated to Steve Jobs in 1993 by