[QUESTION] What's the ":desk" link parameter?

[stardiviner]

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I found some examples setting org-link-parameters with ":desk", like this:

#+begin_src emacs-lisp
(defun org-id-link-desk (link desk)
  "Description function for id: link."
  (let ((id (cadr (split-string link ":"))))
    (org-with-point-at (org-id-find id 'marker)
      (s-join "/" (org-get-outline-path 'with-self)))))

(org-link-set-parameters "id" :desk #'org-id-link-desk)
#+end_src

But I have not found any mention in help of variable ~org-link-parameters~. 
Also I
checked org mode source code by search, no matching of ":desk" found. Did I miss
something?

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