Currently RCV allows sharing of full screen or a given app. However, when user has a high resolution monitor (eg 4k/8k), sharing fullscreen means the viewers may not be able to see the screen clearly as it's downscaled to say a FHD display. On the other hand, sharing only 1 app at a time means user has to unshare and share diff apps if their requirement needs multiple apps to be displayed.

Enabling part of the screen share mitigates this by
1. Sharing a FHD sized part of screen so that users with all sort of screen sizes are accommodated
2. There is less friction in sharing multiple apps as all the user needs to do is drag the window to the part of the screen thats being shared.

This feature has been in xoom for a good while now.