Rat -- During WWI, the Vickers was used in an indirect fire (or sometimes called map shooting) role to “pepper” an area with plunging fire. The distance of the fire was set from range tables which provided the elevation of the gun in degrees. Range tables were eventually replaced with a slide rule-like device. This elevation was set using the clinometer.
The direction of fire, or deflection, was determined from a map (hence, map shooting) and maintained using a fixed point of aim such as an aiming stake and a deflection bar. The deflection bar allowed the gunner to set and check the direction of the fire (i.e., degrees left or right) relative to something like an aiming stake. The whole process is similar to aiming artillery or mortars. The clinometer and deflection bar were largely replaced for laying the gun during WWII with a dial sight which is similar in function to an artillery or mortar sight. The clinometer remained in use to double-check the dial sight's elevation --- you obviously want to avoid accidently dropping fire onto your own troops whrn firing over their heads. One of the reasons the Vickers was obsoleted was that its indirect role was completely replaced by the 81mm mortar by the early 1960s. I might add that the 1917 and 1919 Browning could be used in a similar role, however, US tactics did not seem to emphasize this use. Such plunging fire was called “Hard Rain” which is the title of Frank Iannamico's book on the Browning.
Hope this adequately explains it. This all bring me back to my days of being an 81mm platoon leader.