Uppercase S in permissions of a folder

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http://unix.stackexchange.com – What does the letter S mean below? The file in question is a folder.                                                                 I read here that an upper-case S can represent that the setgid bit is active for a binary executable. But this is a folder. Does it still mean that the setgid bit is activated for it? If so, what does that mean? (HowTos)