Construction lines are very lightweight. A thin, hard pencil creates lines that are grey in color and thinner than the actual object lines.
They have the same appearance of the lightweight lines used in sketches.
Construction lines are used to connect the corners of the object to its vanishing point. the vanishing point is the imaginary point on the horizon where the lines from each corner will eventually converge.
Vanishing points assist in creating perspective, or 3D drawings.
Technical drawings are considered to be "exploded," which means they depict the object from all sides. Construction lines are used to keep each view of the object in line with the view beneath it to make the images easier to imagine as a part of the 3-dimensional whole.
For example, the top view of the object would be drawn above the front view but between two construction lines that run from the outer edges of the front view to make it is easier to imagine the top view as part of the object.
Construction lines are used to signify when a portion of another view of the object may be hidden when looking at another view. For example, if the top view of the object shows a shallow depression in the center, this would not be seen when looking at the front view.
A dashed construction line is drawn on the front view to communicate the presence of the depression in the top view.