To organize your source code into logical units, use subprojects. A subproject is always a child project of a program or another subproject. The source files contained within a subproject are compiled and linked into its parent program; subprojects do not get built into executables. You define a subproject as a special type of build file (*.bld) that does not get built.
If your project uses sub-directories to organize your source code in your file system, you can create the subproject's build file in the appropriate sub-directory. This allows the Builder to look in the correct directory when you add source files to the subproject.