More documentation in https://github.com/emacs-mirror/emacs/blob/7cb77385d38b96377...

The directory is lisp/international, that already tells something

   ;; CCL is used for code conversion at process I/O and file I/O for
   ;; non-standard coding-systems.  In addition, it is used for
   ;; calculating code points of X fonts from character codes.

   $ find /usr/share/emacs/26.3/lisp/ \! -name ccl.el.gz -name \*.el.gz -exec zgrep ccl- {} +
   /usr/share/emacs/26.3/lisp/obsolete/pgg-parse.el.gz:(when (fboundp 'define-ccl-program)
   /usr/share/emacs/26.3/lisp/obsolete/pgg-parse.el.gz:  (define-ccl-program pgg-parse-crc24
   /usr/share/emacs/26.3/lisp/obsolete/pgg-parse.el.gz:      (ccl-execute-on-string pgg-parse-crc24 h string)
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:specified by the attributes `:ccl-decoder' and `:ccl-encoder'.
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:`:ccl-decoder' (required if :coding-type is `ccl')
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:VALUE is a CCL program name defined by `define-ccl-program'.  The
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:`:ccl-encoder' (required if :coding-type is `ccl')
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:VALUE is a CCL program name defined by `define-ccl-program'.  The
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:        '(:ccl-decoder
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:          :ccl-encoder
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:    (plist-put properties :ccl-decoder (car flags))
   /usr/share/emacs/26.3/lisp/international/mule.el.gz:    (plist-put properties :ccl-encoder (cdr flags))))
   /usr/share/emacs/26.3/lisp/language/vietnamese.el.gz:;;  '(ccl-decode-vps . ccl-encode-vps)
   /usr/share/emacs/26.3/lisp/language/ethiopic.el.gz:(define-ccl-program ccl-encode-ethio-font
   /usr/share/emacs/26.3/lisp/language/ethiopic.el.gz:(setq font-ccl-encoder-alist
   /usr/share/emacs/26.3/lisp/language/ethiopic.el.gz:      (cons (cons (purecopy "ethiopic") ccl-encode-ethio-font) font-ccl-encoder-alist))