Your assurance of perfect alignment. The jaws actually float to permit the cartridge to find its exact center in the die.
When a case is in the shell holder, it's supported by spring-loaded jaws that contact the base of the cannulure for more positive extraction. The shell holder is also self acting, so when a cartridge is placed in the die and the handle of the Co-Ax Press is activated, the jaws can open and close on the cartridge case. When the handle is raised, the jaws automatically open and release the case.
-"S" (Standard) jaws are factory installed and ready for reloading
-Optional "LS" (Large/Small) jaws, available separately, accommodate a wide range of very large and small case heads.
An alternative for other calibers is the Shell Holder Adapter Plate for Co-Ax Press, though it does not allow a cartridge to "float" as with the "S" and "LS" Shell Holder Jaws.