I found a strange circular design on my Nikon D3300. I could not find this referenced in the manual. What does this signify?
On page 1 of theNikon D3300 reference manual the [+/-] button pictured to the lower left in the image included in the question is listed as the:
When shooting in manual exposure mode with Nikon cameras that have only a single control wheel, such as your D3300, the wheel - which is labeled the 'command dial' in your user's manual - controls the shutter speed setting . To control the aperture setting you need to press and hold down the exposure compensation button (the button with the [+/-] on it) while turning the control wheel. The 'circular design' is supposed to look like a lens' aperture iris and is placed next to the [+/-] button because it is additional labeling for the [+/-] button.
It's on page 109 of the Nikon D3300 Reference Manual.