The B1 nut has a groove over the top of the threads to accommodate a 3/4" Titanium screen and therefore is optimized to work with the 22mm Screen Diffuser or Injector Diffuser so the balls are secured inside the sub assembly.
The 22mm diffuser is the standard diffuser that works only with the shovelhead bowl. It allows the heat to get closer to the load and you can also preheat the titanium bowl for some extra punch. The 22mm diffuser wraps around the outside of the shovelhead bowl.
The injector diffuser rests on the inside of the shovelhead bowl. It also works with a number of other injector bowls on the market so it's a bit more versatile considering it still works with the shovelhead.
Choose no filler if you have your own or select from 3mm boro, 3mm ruby or 4mm ruby. Ruby has different thermal properties than boro. It will allow for a lower operating temperature and recover quicker after draws. 3mm is going to provide slightly more draw restriction than 4mm.