These give you the most predictable lines. They are filled with pigment drawing ink, either from a bottle, or using cartridges. The nib sizes of these pens are quoted in mm, and represent the line width you can expect, unlike the relatively arbitrary numbering of disposable fibre-nib drawing pens.

These are the easiest types of pens to use and maintain, because you just throw them in the bin when you're done. They are cheap to buy, but can work out expensive in the long run if you use them heavily, because replacing the whole pen is generally more expensive than replacing a refill, or just refilling with ink.

The nib sizes of these pens is given as a number but these aren't an actual line width in mm. It's more a relative thing. Some brands state the line width separately, others don't.

With some of these, you replace the refill and nib unit in one - the whole centre of the pen is the refill. With others, you can replace the nib units separately - more fiddly to do, perhaps, but you don't need to replace perfectly good nibs, and if the nib wears out before the ink, you're not throwing away usable ink.

Nib sizing is variable - some are simply relative numeric, others specify exact line width.