Section 27.3 Numeric modes contains:
Here is how the bits are arranged, starting with the lowest valued bit:Value in Corresponding Mode Mode Bit Other users not in the file's group: 1 Execute/search 2 Write 4 Read Other users in the file's group: 10 Execute/search 20 Write 40 Read The file's owner: 100 Execute/search 200 Write 400 Read Special mode bits: 1000 Restricted deletion flag or sticky bit 2000 Set group ID on execution 4000 Set user ID on execution
These correspondences assume that the file is a regular file. For example, for a directory (where execution doesn't make sense), 2000 causes files created in the directory to inherit the directory's group. http://en.wikipedia.org/wiki/Setgid#setuid_and_setgid_on_directories has more information.
