Use the lightning:availableForChatterExtensionComposer interface to integrate your custom
apps into the Chatter publisher and place the custom app’s payload in the feed. This
interface has an effect only in Lightning communities.
The lightning:availableForChatterExtensionComposer interface
works with the lightning:availableForChatterExtensionRenderer interface and the lightning:sendChatterExtensionPayload event to integrate your
custom apps into the Chatter publisher and to render the app’s payload in the feed.