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The first 31 characters or so in the ASCII set are reserved as "control" characters. The most famous would probably be the null character and the escape (^e). These characters were intended not to be seen, but to give invisible commands to whatever entity is handling the file with the controls.

These days attempting to open a file with control characters (usually a non-ascii or binary file, which just happens to contain byte values between 0 and 31 and doesn't care these mess ASCII up) will result in either the control characters screwing things up in some unpredictable way, the control characters being ignored, or the control characters being mutated into boxes or lines or whatever your OS chooses to make them. Less tends to display them as an inverted block containing a ^ followed by the characters raw byte value as a decimal (as in base 10) number.

In HTML, they will let you insert any raw ASCII value you want by typing an & followed by a # followed by the number value followed by a semicolon. In other words � would be the null character.
Do not try to make Everything nodes whose names are control characters. The editors seem to have a policy of nuking nodes with control chars in the names.

See also the excellent writeup at control codes

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