Microsoft announced today that along with Timeline, the upcoming builds of Windows 10 "Redstone 4" will receive a new feature, provisionally called "Sets". It is the implementation of the tabbed shell for Windows 10 which will allow app grouping exactly like tabs in a browser.
When enabled, Sets will allow combining windows from different apps in a tabbed view. Just like you can switch between web pages with tabs, each open tab will represent a running app.
Sets will sync between all your devices and will be integrated with the Timeline feature, so it will be possible to quickly restore a group of apps on another device assuming those same apps are available on that platform.
The idea behind Sets is to provide the ability to conveniently group apps for a task you are doing in one window. For example, you could arrange all your running Microsoft Office apps in one group.
Currently, the Sets feature seems to be limited to Store apps only. This means you cannot put classic desktop apps into groups. The company will add support for Win32 apps in early 2018.
The Sets feature will become available in upcoming builds of Windows 10 "Redstone 4" Insider Preview. However, it will be rolled out to select users because for features that have not been finalized, Microsoft often does A/B testing. Certain options and features are only available to a lucky few Insiders. So, don't be surprised if you don't get the Sets feature immediately in the Preview build. When the final version of Windows 10 Redstone 4 ships, Sets should become available to everyone.
Sets are expected to be customizable. It will be possible to exclude certain apps from Sets, so they are never included in a group.
Note that Sets is a tentative name, which Microsoft may change in the future.