Source Insight automatically creates a project named Base. When Source Insight cannot find a symbol definition anywhere else, it searches the Base project. It is implicitly at the end of the project symbol path.
You do not have to add the Base project to the project symbol path. Source Insight automatically searches it as though it were in the list.
This gives you a convenient place to save common symbols. Any symbol stored in the Base project is visible from any other project. The Base project is also a good place to add your favorite Source Insight editor macro files.