Crossposted videos is a content management capability to reuse the same video across multiple posts and / or Pages.
The associated Insights updates give a view of total performance across these multiple users of the same video.
Create a new post with an existing Video
Once permission to reuse a video has been granted, any Page in that same Business Manager can create new posts with the video without having to re-upload it.
See total performance insights
If a video has been crossposted, Page owners of the Page that initially uploaded the video will be able to see metrics for each post in which the video has been reused, as well as aggregate metrics for the video across all those posts.