Allow a user to author a post accross multiple blogs in the same Multi-Site install.
Multi Posts will allow you to post to other sites on the same WordPress install that is configured to support multiple sites. It will not let you post to other sites or blogs.
Multi Post only allows you to post to other sites that also have Multi Post turned on. As the site's administrator to turn it on.
When you update a post, the changes will go to all other sites it was posted to. (They are selected in the Multi Post metabox on the right.)
When you deselect a site from the Multi Post metabox, you are given the option to delete or orphan the post in that site. When you update the post,