It will host a session during the Games Developers Conference in San Francisco in March in which is will announce a new SDK for iOS, Android and Nintendo Switch. This will enable games developers and studios to add Xbox Live support in their titles for those platforms.
The full benefit of that will be unveiled at the time, but the GDC diary description states that Xbox Live would "handle social, communication and multiplayer interactions across billions of screens".
That could potentially mean that players on iOS, Android or Switch earn Xbox Achievements and score when playing on those platforms, chat via the numerous Xbox community systems and, most importantly, have seamless cross-platform play across Xbox One, PC, iOS, Android and Switch.
However, the new SDK will enable third-party developers to add cross-platform play without having to deal with the logistics of it; Microsoft will handle that end.
We'll find out more when GDC 2019 starts on 18 March.