Pen and ink has no grey. A nib either touches the paper or it does not, so every tone in a drawing is a count: how many lines cross this square millimetre, and at how many angles.
Building tone in passes
An illustrator works in layers. The first pass covers everything above the lightest tone with parallel strokes in one direction. The second pass runs over the darker areas at a crossing angle. The third and fourth keep going until the deepest shadows are nearly solid.
This tool works the same way. Each pass owns a band of the tonal range, and a pixel only receives a pass if it is dark enough to have reached that band. A light area gets one direction, a shadow gets all of them stacked.
That is why raising the pass count deepens the blacks rather than thickening the lines. More layers, not heavier ones.
Line Spacing and Nib Weight
Spacing is the pitch between strokes, set as a percentage of the shorter image side so the preview and the exported file draw at the same density. Nib Weight is how much of that pitch the stroke fills.
The two together decide whether the drawing reads as line work or as a grey wash. Wide spacing with a thin nib leaves visible white paper and looks drawn. Tight spacing with a heavy nib fills in and looks printed. If the midtones are turning muddy, widen the spacing before thinning the nib.
Hand Wobble
A ruled line reads as machine output no matter how good the tonal mapping is. Wobble displaces each stroke along its own axis using smooth noise, so lines drift, converge slightly, and separate again the way a nib does across textured paper.
Between 25% and 50% is where most drawings sit. Above 70% the passes stop reading as parallel and the image turns to scribble.
Contour Line
Hatching carries tone. It does not carry edges, because a boundary between two similar tones falls inside the same band and gets the same treatment on both sides.
Contour Line adds an explicit stroke where the local tone changes fastest, which is what an illustrator does first and fills in around afterwards. On portraits and product shots it is usually the control that makes the subject legible.
Limits
Tone comes from luminance, so a saturated colour and a neutral of the same brightness receive identical hatching. A red object against a mid-grey background will vanish. Separate them with a channel mixer or a black and white conversion before drawing.