What do "dashes" mean in Permissions-Tab of file/folder properties?

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http://askubuntu.com – If I use dconf-editor to change the way permissions display (how to change permissions appearance in 12.04) and then bring up the file/folder properties of something in nautilus (e.g., right-click a folder and select properties > permissions), sometimes the boxes for permissions under the Permissions-Tab have dashes in them, like so: What do the dashes mean? I was expecting a checkmark/no-checkmark to signify enabled/disabled. But dashes? (HowTos)