A 2D View shows the Top, Front, or Side View of an object.
By changing your view, you will see different 2D shapes.
Orthographic Projections - Miter Line
The miter line is a technical drawing convention used to assist in the creation of orthographic projection drawings. The miter line is a construction line that begins at the apparent upper-right corner of the front view and extends indefinitely at a 45 degree angle. The line is used to help create the top and right side views in third-angle construction by allowing you to make visual connections between the views. Any feature in either the top or right side view can be projected to the miter line and then angled 90 degrees towards the other view.
Orthographic Projection - Hidden Lines
A hidden line is a technical drawing convention used to assist in the creation of 2-dimensional orthographic projection drawings. The hidden line is a dashed line that shows a feature or surface that is hidden within that view's vantage point, but necessary to help the viewer more easily visualize the 3-dimensional object when all views are considered. Hidden lines in one view will relate to visible (object) lines in a different view.