These are supposed to solve an issue where due to different thermal coefficients of linear expansion between steel (rails) and aluminum (extrusions), the Voron V2/Trident gantry forms a sort of giant bimetal temperature sensor element. This causes a warping of the gantry as it heats up with increasing chamber temperature.
These are approximately 3.2mm thick, order based on the size of your build x/y build volume at a 1:1 ratio (e.g. a 250mm Trident would order 250mm version of backers). This comes in sets of 1 specifically for the CAP Elite X Beam (found here). We include titanium screws!