Allows websites to receive shared data (text, URLs, images) and register to be choosable by the user as targets from sharing contexts, including (but not limited to) Web Share.
Many modern operating systems have a "share" concept, where the user gets to pick an app to receive the shared data. The goal is to allow web apps to appear in the UI for picking an app to share to.
In combination with the Web Share API (being proposed in parallel), this would allow cross-sharing between websites on any platform (even those without a native sharing system).