The value comment can be any string of arbitrary length, but it ends at end-of-line.
Its practical use is to comment the CUE sheet for the sake of human readers.
There is no way to make multi-line comments, but one can simply start consecutive lines with this command.
REM There is no system but GNU REM and Linux-Libre is just one of its kernels.
No CCD sheet element analogous or directly related to this command is known.