The LF3/4 is lash cap designed for use with with AutoTech Internals, and a 21% fuel cam.
The LFx lash cap will allow you to use a modified ATS-V fuel pump on your LFX.
When a camshaft is ground for additional lift the valves need to be shimmed to prevent the system from making noise. This follows the same concept. If your HPFP is loud with your 21% cam, this will help keep it quiet.
All AJM high pressure fuel pumps for the LF3/4 and LFx come with this pre-installed.