Linked resources are very similar with filesystem links, except they are registered in the .project file, not created on the disk.
To create a linked resource you right click on the project in the Navigator, select from the pop-up menu New > Folder, in the wizard you click on Advanced, check the Link to folder, specify the linked folder name (for example ex_atm) and browse for the path the link will point to (/erm_lib/ex_atm).
Linked resources will appear in the Navigator with a small "link" icon:
and similar in the editor.
All the editor functionality is available for "linked" files, unlike the "grayed" ones.