Consider the case when you have a set of header files from a library that you often use with multiple projects. You could add these files to each of your projects, but that would be redundant. A better solution is to create a single common project for each set of common header files, and let Source Insight search the common project from any open project.
Common projects enables you to create smaller, self-contained projects, but still have the ability to locate symbol declarations in other projects. The Base project (See: The Base Project.) is the final place Source Insight looks to find symbols; it is implicitly at the end of the project symbol path.
Common projects are used in two ways:
You can add the common project as an import library project. There global and project-specific import library lists. See: Importing Symbols from Libraries.
You can put the common project onto the project symbol path. There is only one project symbol path that is used regardless of what the current project is. See: The Project Symbol Path.
Source Insight will search the import library projects, and the project symbol path whenever it looks up a symbol and can't find it in your current project, or in an open file.