A Stub IOS is an IOS that serves no purpose other than a placeholder. They are typically released by Nintendo in order to overwrite any existing version of an IOS and render it unusable for piracy or homebrew purposes.
Most stub IOSes are version 65280; this is version 255.0. Most homebrew IOSes use version 65535 (255.255) to prevent Nintendo from updating them.
Stub IOSes generally have 3 contents: a 0x40 byte header content, and 2 shared contents that vary between IOSes. The selection and purpose of these shared contents is unknown.