There are two different projection modes: perspective and orthographic. Perspective projected views are calculated based on the distance from a theoretical camera and target point. Orthographic projected views display all the points of a model being projected parallel to the screen.
Perspective projection mode
In Perspective Camera mode, part or assembly models are displayed in three-point perspective, a visual effect in which parallel lines converge on a vanishing point. This effect is the way real objects are perceived by the human eye or by a camera.
The shorter the distance between the camera and the target point, the more distorted the perspective effect appears; greater distances produce less distorted effects on the model.
Orthographic projection mode
In orthographic projection mode, a model is displayed so all its points project along parallel lines to their positions on the screen.
All same-length parallel edges display as the same length, even when you orient them so one edge is closer to you than the other. In Orthographic Camera mode, a 3D model appears flat and unlike objects observed in the real world.
When the view for a model changes, the view is updated using the last projection mode unless the current projection mode set for the ViewCube is Perspective with Ortho Faces. The Perspective with Ortho Faces mode forces all views to be displayed in perspective projection unless the model is being viewed from one of the face views: top, bottom, front, back, left, or right.