Projects can be linked. In a linked project, the source files are those from the linked project and are shown with an (l) suffix. In other respects they are treated as if they are normal source files. Other files in a project are not linked.
This lets you work on two or more applications that both use the same source in their projects. With project linking, you can create the source in one project, and link to it from the other projects.
If you work on several applications, it can be useful to build a core library of utility projects that can be included as required in each application. In this case, to include any of the utilities, link to the utility projects from your application project.