Interchangeable bushings, available in one-one thousanths of an inch increments, allow sizing the case neck for proper grip on the bullet without having to expand or bell the neck.  To have a working die, you must choose a die body to fit your cartridge, either decapping top or an open top to hold the bushing in the body, and one or more bushing diameters.  

Measure  the diameter of several LOADED cartridge necks.  Subtract 0.002 to 0.003 inch from the neck diameter.  This should get you close to the bushing diameter you will need depending upon the amount your case necks spring back and how much neck tension you feel you  need.

You will need to choose at least one bushing to be able to use the die. 

Screws into the top of the die body and holds the sizing bushing in place.  This top also includes a de-capping rod and can not be used crimp loaded cartridges.  You must have one of the two tops to have a funtional die setup.

Screws into the top of the neck sizing die body  to hold the tapper crimp bushing in place but this top is bored through to allow the bullet of a loade cartridge to  extend up through the bushing and the top of the die. You must have one of the two tops to have a funtional die setup.