There are taper, plug, and bottoming taps for each size. The plug (or intermediate) works alone for most materials in pen making. Some people use the bottoming tap after if using a transparent material to get cleaner looking threads internally. I am not really sure what the taper tap is used for.I'd probably be interested as well.
Quick question though - why are the same due types listed more than once in the example - is there more info to the left that differentiates the line items?