The projection mode tool 
can be found in the toolbar when a 3D view is active.
This tool is used to change the way in which the 3D graphics are projected (it has nothing to do with the view or layer projections). In other words, the 3D views can be projected basically in two ways: orthographic or perspective. This tool changes between these two types of projections.
Note: When the orthographic mode is active it is not possible to zoom in the view. There are only the options of panning or rotating.
