manual/include/automation-modes.html

<p>
  Ardour offers two modes for connecting automation control points: <kbd
  class=menu>Discrete</kbd> and <kbd class=menu>Linear</kbd>. The mode is
  changed by a right click on the automation lane header and choosing the
  mode from the mode menu.
</p>

<p>
  <kbd class=menu>Discrete</kbd> mode has stair steps between each point on the
  automation track. This is useful for on-off automation like mute or sustain
  pedal (on a piano MIDI track).
</p>

<p>
  When recording automation via MIDI (for example a pitch bend from a keyboard),
  Ardour always uses discreet mode.
</p>

<p>
  <kbd class=menu>Linear</kbd> mode has straight lines betwen each point on the
  automation track. This is useful for gradual shifts in automation lanes,
  such as gradual increase in volume on the fader.

  Linear is the default mode for most automation lanes created via mouse input
  (versus recording via MIDI).
</p>