The library can be viewed as a storage place for third-party elements such as:
All those elements are first placed in the library and can then be re-used in any number of topics within the current project. Once a library item has been placed in the topic editor, it is then linked with the library item and therefore any modification made to the library item will also be made to the linked elements. As an example, changing a picture in the library which has been placed in hundreds of topics, will automatically change all those topics with the new picture.
To insert an item in the library, from the "Home" tab use the "Add item" button from the "Library" group. To insert an item from the library to the topic editor, either: