This is an alert code to the computer or typesetter to tell it that the character or characters following it are to be interpreted as instructions or as special alphanumeric rather than their normal meaning. For instance Bell L tells the machine that this sequence of characters is for a line length and not for the character L. Sometimes these codes are called "Flag" or "Super Shift" codes so as to distinguish their purpose more clearly. Super Shift codes are usually used to encode characters that are not available as part of the keyboard layout, whereas precedence and flag codes are more for commands.
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