is for Emacs display features, which includes scrolling, fonts, colors, and placement of text or images.
Tag display refers to how text and other visual objects appear in Emacs. Emacs has many commands and settings that affect display. They are summarized in the manual. This includes highlighting, the fringe, whitespace, text wrapping, cursor and mouse-pointer appearance.
Some display features have their own tags, such as faces, fonts, font-lock, scrolling, narrowing, mode-line, and text-properties.