Viewport Toolbar

On the left side of the viewport, there are a few icons which will change based on which sidebar mode that you are in. The two which do not change are the “camera” icon and the “eye” icon. (The other icons are covered elsewhere)

The camera icon switches the camera’s projection view, identically to how you can do it in the “View” menu, or by pressing Numpad 5.  

The eye icon adjusts the visual settings for the viewport, allowing you to change scene transparency, grid transparency, and ray transparency. You can also adjust the visual “cutaway” of the 3D scene, allowing you to see directly inside the model with unwanted pieces of geometry being “culled” (invisible).