Recently, Google renamed Chromecast to Cast to make it a bit more uniformed and keep it away from Chrome since it doesn't actually run on Chrome, and they've been looking to expand the Casting functionality past just media onto the big screen. We've seen the ability to play games on TVs, and we've even seen the ability to Cast Chrome Tabs to other cloud-based apps like Hangouts. It looks like this feature, which actually appeared as long ago as May 2015, is now showing up for some users. However, it's not actually working for the users that are seeing the image shown above.
Perhaps this means that Hangouts won't be resting in peace after Allo and Duo are finally available later this summer. Being able to Cast a Chrome Tab in Hangouts could be a pretty cool feature, and Google is likely referring to video Hangouts here and not your standard text messaging or instant messaging, which many use Hangouts for currently. However, what is interesting about the image above is that it refers to "Cloud services" like Hangouts. Perhaps we'll see this functionality for other cloud services too? It wouldn't be surprising to see it come to Duo when that launches, but it's definitely interesting to see the wording there.
The ability to Cast a Chrome Tab into Hangouts could be pretty useful actually. Imagine that you are in a video Hangout and want to show the participants something. Currently, you'll need to show your entire screen, or the entire app like Chrome. With this Casting feature, you should be able to Cast just a single tab. And it also sounds like you'll be able to Cast from a separate device from the one you are currently using Hangouts on, since it's doing so over the cloud and your internet connection. Probably not the most popular feature that Google has come up with for Casting, but it could prove to be fairly useful. Currently, it appears that this is only showing up for those on the desktop version of Chrome. And it's not appearing for everyone right now. It's likely a staged rollout, as most things are with Google these days, especially since they want to test things out before rolling it out to everyone.