Segment Selection Actions
 
Click + Select - Clicking on a segment highlights and thus selects it. Selected segments will be become red, but the endpoints will remain hollow.

Drag + Select - This action allows selection of multiple points and/or segments. You must drag a selection over the entire segment, including its endpoints. All selected segments will turn red as will their end points.

Shift + Click + Select - This action allows you to add (select) or subtract (deselect) segments from an active/selected group. All selected segments will turn red, and the points associated with the segments will become solid/active.