This functionality is available to professional users and above.
This allows you to split a post into two separate posts. This is mainly for threads, if part of a post should be moved to a different thread.
If you created the thread, you can split any of the posts in that thread, including those made by other people. But you need to have at least adder rights to the post, which is the default for threads. Also, if the post has already been cross-posted to another thread, you can't split it, because you don't know what impact it would have on the other thread.