Erector Tube
The internal tube inside a riflescope that carries the erector lenses and reticle; turret adjustments move it, and its travel limit sets the elevation ceiling.
Inside a riflescope sits a smaller assembly called the erector tube, which holds the lenses that flip the image upright and, in most designs, carries the reticle as well. When you turn the elevation or windage turret, you are physically pivoting this tube within the main body, shifting where the reticle points relative to the bore. A spring holds the tube against the adjustment screws so that every click moves it a precise, repeatable amount.
Because the erector tube can only swing so far before it bottoms out against the main tube wall, its range of motion defines the total internal adjustment available. That hard limit is what the scope elevation ceiling describes, and it is a central concern when pushing a cartridge to extreme distance, since running out of erector travel means running out of dialed elevation no matter how much drop remains.