^ Exactly. Cobb's own personal dream construct used the elevator image to trigger a mental shift in the different levels. His were more personal though and the elevator fits that. But when they are on business, the use of the dream initiation box is a clear concise image to everyone involved. For the most part is the perfect representation of that mental trigger to all parties.
And I had never thought of River's experience with the gun in this context but it really does work as a visual example of this. To her it was an ordinary object and didn't mean the same thing as it did to everyone else. And come to think of it, her statement about the cows remembering who they were once they were outside again does too in a way. They had no meaning or purpose until they were where they were supposed to be in the context of their own reality. (River herself ended up being much the same way in the end.)