A User Library in Eclipse is a set of jar files.
In the classpath file, you can read the following path org.eclipse.jdt.USER_LIBRARY/MyLibraryName.
<?xml version="1.0" encoding="UTF-8"?>
<classpath>
<classpathentry kind="src" path="src"/>
<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>
<classpathentry kind="con" path="org.eclipse.jdt.USER_LIBRARY/Oracle JDBC"/>
<classpathentry kind="output" path="classes"/>
</classpath>