From the manufacturer:
There are two different kind of nibs, broad-edge and pointed, which can be inserted into Speedball Pen Holders.
When lettering Italic, Roman, Blackletter, etc., select a broad-edged tool like the C-Series Speedball nibs. When lettering pointed pen hands, such as Copperplate, use a flexible pointed nib such as the Speedball 99 or 103. For monoline lettering, select a Speedball B-series nib.
Click HERE for Pointers for Using Speedball Pen Holders and Points