A COBOL copybook is used in programs that are in 2 or more projects, and those projects are dependent on the project containing the copybook.  This dependency is set by using the "build path" in the properties of each project.  

All the projects are in the same Eclipse workspace.

When the copybook is selected and the menu-option "Compile Dependent Programs" selected, not all of the programs will be compiled.

Full article: https://portal.microfocus.com/s/article/KM000015254