New in version 3.7.
This read-only directory property contains a semicolon-separated list of subdirectories processed so far by the add_subdirectory() or subdirs() commands. Each entry is the absolute path to the source directory (containing the CMakeLists.txt file). This is suitable to pass to the get_property() command DIRECTORY option.
Note
The subdirs() command does not process its arguments until after the calling directory is fully processed. Therefore looking up this property in the current directory will not see them.
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https://cmake.org/cmake/help/v3.19/prop_dir/SUBDIRECTORIES.html