Library layering using prerequisite libraries allows general purpose or baseline items to be shared across multiple project or extension libraries.
Each prerequisite library forms a layer in a virtual library composed of all prerequisite layers and the working layer. The library that you open becomes your working layer, and its prerequisite libraries correspond to the prerequisite layers.
Here are the simple steps to add a prerequisite library (with all of its content) to an existing library
1. Check out and open your library
2. Open the Libraries Properties editor
3. Click the second tab 'Prerequisites'
4. Click the '+' to add a new perquisite
5. Choose the library you wish to be the prerequisite by selecting the ellipsis [...] to the right of the library name
6. Find the library you want and click OK to confirm your selection
7. Click OK to save the updated library properties
Note that the updated prerequisite libraries content will not be loaded to your library until you close and reopen your library.