Please forgive my ignorance, but what is the difference between redding's 2 die & 3 die, die sets?

The two die set is the FL (Full Length Resizing) and the ST (Seating Die). The three die set is the FL, ST, and the NK (Neck Sizing Die).

Or, full length sizer, belling die, and seater/crimper if in a straight wall pistol bullet.

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