Control-character lines are used to display characters whose code values fall outside the range of printable characters from US ASCII ( Ecma-6 / ISO 646). That is, those characters whose code values fall below below 32 decimal (those from the C0 set of control characters from Ecma-48 / ISO/IEC 6429); and those whose values are at or above 127 decimal (the “delete” character, and character byte values with the high bit set).
Control-character lines begin with an initial dot, ‘.’, followed by the control characters or high-value bytes being represented.
. BEL/^G NUL/^ CR/^M LF/^J DEL/^? xA0 xFF
Control characters (whose numeric codes fall below decimal 32, plus the delete character at decimal 127) are represented by a mnemonic acronym identifying the character’s function. Unless the -C was given, this acronym is followed by a slash, and the control-key combination that would produce the corresponding character (control characters are usually much more recognizable from one or the other of their name or their control-character representation, than they are by their hexadecimal code value). The “control-key combination” representation consists of a “hat” or “circumflex accent” character, followed by a character with a value in the range of 63 through 95 decimal.
Note that the delete character, designated as ‘DEL/^?’, is a special case, in that one can not generally reproduce that key by holding down the control key and typing a question mark; it is simply used as an identification of the key.
Other values (high-value bytes) are represented by the lowercase letter ‘x’ followed by the two-digit hexadecimal code value for the character.
For reference, here’s a table of the control characters (plus DEL). It is based on the information from Table 1 of Ecma-48 / ISO/IEC 6429 (the control-key representation has been added).