Many common measurements, like measuring a door or window rough opening, or across foundation forms to check square, are diagonal measurements that require tetting the Laser Distance Meter into a corner.  The problem is that the thickness of the base of some laser distance meters means you can't get it al the way into the corner, so your measurement will be off by the distance between the back of the tool and farthest reach of the corner. The solution is a flip-out extension, that is standard on all Leica Disto's E7300 and E7400x , that will fit all the way into the corner.

The Leica Disto E7300 and E7400x not only has a flip out extension, but this extension when flipped out is connected to the units internal circuit board.  That is why when you flip out the end piece, it tells the device that the distance measured will be corrected in the unit so the distance measured is now corrected by the end piece flip out and you do not have to think about it.