A useful article and a sample project about how to copy a bookmark into another document is here: http://www.aspose.com/Community/forums/ShowThread.aspx?PostID=65476#65476.
A document in Aspose.Words is a tree of nodes and a bookmark in a document logically defines beginning and end of a set of different nodes, including paragraphs, tables, runs, sections etc, all depending on where the bookmark begins and ends.
Some tasks with bookmarks are easy to perform with Aspose.Words’ Bookmarks, Bookmark, BookmarkStart and BookmarkEnd classes. You can find, enumerate, create and delete bookmarks. You can also get or set plain text of the bookmark in a string.
But at the moment there are no public methods in Aspose.Words to copy complex content of a bookmark with formatting (paragraphs, tables, etc) into another place or into another document. The above mentioned article provides a code sample how to do that with a bit of coding.