It may be documented in chapter 4, entitled Hyperlinks.

   http://orgmode.org/org.html#Hyperlinks

I don't know if this chapter describes export behavior.

You can skim through org-latex-link if you care.  For headlines this
is the relevant part

;; LINK points to a headline.  If headlines are numbered
;; and the link has no description, display headline's
;; number.  Otherwise, display description or headline's
;; title.
(headline
 (let* ((custom-label
         (and (plist-get info :latex-custom-id-labels)
              (org-element-property :CUSTOM_ID destination)))
        (label
         (or
          custom-label
          (format "sec-%s"
                  (mapconcat
                   #'number-to-string
                   (org-export-get-headline-number destination info)
                   "-")))))
   (if (and (not desc)
            (org-export-numbered-headline-p destination info))
       (format "\\ref{%s}" label)
     (format "\\hyperref[%s]{%s}" label
             (or desc
                 (org-export-data
                  (org-element-property :title destination) info))))))