If you were building products to run on multiple operating systems, you'd include an OS-level directory structure. This would serve as a branchpoint for OS-specific subdirectories.
In our factory-floor example, the gui section might be built for both QNX 4 and BlackBerry 10 OS, whereas the other sections might be built just for BlackBerry 10 OS.
If no OS level is detected, BlackBerry 10 OS is assumed.