Why the $bytes identifier? confused

Below's but a quick scribble - I'm sure you can find some elaborated calculation scripts at the various dedicated mIRCscript websites.
on *:text:!calc*:#: {

  ; no parameter given: syntax help
  if ($2 == $null) { msg $chan $nick $+ $chr(44) try: !calculation <stuff to calculate> }

  ; parameters don't contain any digit or "pi"
  elseif (!$regex($2-,/(?:\d|pi)/)) { msg $chan Nothing to calculate, $nick $+ ! }

  ; calculation output, with x replaced by *, commas by dots, pi by it's value
  ; note that it isn't supporting $sqrt, $sin, $cos, $tan etc as yet, and odd bracket usage may break the calculation, like in " 5+2*(3 "
  else { msg $chan $nick $+ $chr(44) the answer is: $calc( $replace($2-,x,*,$chr(44),$chr(46),pi,$pi) ) }