The CDI button toggles the navigational source between VLOC and GPS modes. This
specifies which type of navigation is being shown on the indicator.

Directly above this button on the display, the letters "GPS" or "VLOC" are displayed to
indicate the current mode selection.

This button is replaced by a NRST button in the GPS 400 and GNC 420 receivers  because
they have no VLOC (Nav) radios.