There are 2 different editing modes for operations which affect how the insertion point and editing on the timeline operate. There is: drag and drop mode and cut and paste mode. The editing mode is determined by selecting the arrow , or immediately to the right of the arrow, the I-beam in the Transport and Buttons bar. In figure 2.4 you can see the green colored highlight 1 on the arrow icon indicating that you are currently in drag and drop mode.
With the arrow highlighted for drag and drop mode, a double click with the left mouse button in the timeline selects the edit the mouse pointer is over. Then dragging in the timeline repositions that edit and this can be used for moving effects, changing the order of playlists, or moving video pieces around. There are numerous methods to cut and paste in drag and drop mode by setting In/Out points to define a selected region or using the Copy/Paste Behavior as outlined in 5.5.1. In this mode, clicking the LMB in the timeline does not reposition the Insertion Point.
When the I-beam is highlighted, you are in cut and paste mode. In cut and paste mode, clicking the LMB in the timeline does reposition the Insertion Point. Double clicking in the timeline selects the entire edit the cursor is over, i.e. that column. Dragging in the timeline with the LMB pressed down, highlights a selected region and this is the region that is affected by cut and paste operations. It is also the playback range used for the subsequent playback operation. Holding down the Shift key while clicking in the timeline extends the highlighted region.