mode-line-display missing?

[Emanuel Berg]

(defface mode-line
  '((((class color) (min-colors 88))
     :box (:line-width -1 :style released-button)
     :background "grey75" :foreground "black")
    (t
     :inverse-video t))
  "Face for the mode lines (for the selected window) as well as header lines.
See `mode-line-display' for the face used on mode lines."
  :version "21.1"
  :group 'mode-line-faces
  :group 'basic-faces)

What is "mode-line-display"?

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