De acordo com a manpage:
-r or --raw-control-chars
Causes "raw" control characters to be displayed. The default is to display control characters using the
caret notation; for example, a control-A (octal 001) is displayed as "^A". Warning: when the -r option
is used, less cannot keep track of the actual appearance of the screen (since this depends on how the
screen responds to each type of control character). Thus, various display problems may result, such as
long lines being split in the wrong place.
-R or --RAW-CONTROL-CHARS
Like -r, but only ANSI "color" escape sequences are output in "raw" form. Unlike -r, the screen appear‐
ance is maintained correctly in most cases. ANSI "color" escape sequences are sequences of the form:
ESC [ ... m
where the "..." is zero or more color specification characters For the purpose of keeping track of
screen appearance, ANSI color escape sequences are assumed to not move the cursor. You can make less
think that characters other than "m" can end ANSI color escape sequences by setting the environment
variable LESSANSIENDCHARS to the list of characters which can end a color escape sequence. And you can
make less think that characters other than the standard ones may appear between the ESC and the m by
setting the environment variable LESSANSIMIDCHARS to the list of characters which can appear.
Aparentemente, less escapa dos códigos de escape ANSI por padrão, exibindo caracteres (^) seguidos por códigos. -r não escapa desses caracteres, portanto, se a entrada contiver dados binários aleatórios, o console poderá gerar um efeito não intencional como resultado de qualquer caractere de controle ANSI. (É por isso que less não sabe como é a tela sem processar esses caracteres em si.) -R permite somente caracteres de controle de cores, portanto, a saída pode conter texto colorido, mas não outros caracteres de formatação que poderiam atrapalhar a saída.