User Libraries provide an ability to extend the standard command library set available in accelQ. Based on your specific need, you may upload multiple libraries. Once the library is imported, all the extension commands become available in the Action logic editor just like standard accelQ commands.
User library is just like any standard java source file with some accelQ specific annotations expected to be incorporated.
This feature comes in handy in case you have existing test logic available in java source files and would like to bring it into accelQ readily. In addition, if you have external teams (not using accelQ) that are developing and managing test logic relevant for your needs, you could import them directly as user libraries.
As a best practice, it is recommended to manage all test logic in Action editor as it makes long term maintenance and traceability easier.