If you specify a display, selects it as the current input
    display. This causes that display to echo PCA input (which always
    appears in the PROMPT display). The display specified must be an
    output display.

    If you do not specify a display, the current input display is
    deselected and PCA input is not echoed to any display (PCA input
    appears only in the PROMPT display). By default, no display has
    the input attribute.