The Double Bar button is only available in Notation view. When enabled, the Double Bar option shows the final double bar line in the MIDI Editor window. The double bar line is located at the end of the last MIDI clip or event in the session.
When the Double Bar option is disabled, there will be a number of empty bars at the end of the last event in the session. The number of empty bars is determined by the Additional Empty Bars In The Score Editor setting in the MIDI Preferences page (Setup > Preferences). Disable the Double Bar option to manually enter notes in the Score Editor after the last MIDI clip or event in the session.