What's the difference between "Add JARs" and "Add External JARs" in Eclipse?

In a project's Properties page, under "Java Build Path" -> "Libraries" page, I can't figure out what the difference is between the "Add JARs" and "Add External JARs" buttons.

Why can I not add external JARs in Eclipse?

If your project builds with Java 9+ make sure you've selected Classpath (as shown here). You should also be able to add the JAR with right-click on the project > Build Path > Add External Archives.... If that doesn't help, would you tell use the version and package of Eclipse IDE you're using?

Add Jar - to include jar files in you build path which are already present in your project.
Add External jar - used to include jar files which are 'outside' your eclipse project workspace folder. They will either be linked or copied.