manual/include/which-regions-are-affected.html

<p>
  This section explains the rules used to decide which regions are affected
  by editing operations. You don't really have to understand them&mdash;hopefully
  things will Just Work&mdash;but it may be useful eventually to understand the rules.
</p>
<p>
  Editing operations in Ardour either operate on a single point in time
  (<kbd class="menu">Split</kbd> being the obvious example) or on two
  points (which can also be considered to be a range of sorts), <kbd
  class="menu">Separate</kbd> is a good example of this.
</p>
<p>
  Most operations will operate on the currently selected region(s), but if
  no regions are selected, the region that the mouse is in will be used
  instead. Single-point operations will generally pick a set of regions to
  use based on the following rules:
</p>
<ul>
  <li> If the edit point is `mouse', then
    <ul>
     <li>if the mouse is over a selected region, or no region, use all selected
     regions, or</li>
     <li>if the mouse is over an unselected region, use just that region.</li>
    </ul>
  </li>
  <li> For all other edit points
    <ul>
      <li>
        use the selected regions <em>and</em> those that are both
        under the edit position <em>and</em> on a selected track,
        or on a track which is in the same active edit-enabled route group
        as a selected region.
      </li>
    </ul>
  </li>
</ul>
<p>
  The rationale here for the two different rules is that the mouse edit point
  is special in that its position indicates both a time and a track; the other
  edit points (Playhead,Marker) indicate a time only.
</p>