The dotted border shows the extent of a basic mask falloff: darktable applies a simple gradient between the solid outline and the dotted line. That transition is geometric and does not consider image content.

Feathering is different: it smooths the mask while trying to align the mask edge with actual edges/features in the image. In other words, it is content-aware, not just a fixed distance fade.

So:

• solid line to dotted line = manual gradient width of the mask edge
• feathering = edge-aware smoothing/refinement of the mask

If the dotted line is close to the solid outline, the normal gradient is narrow. Setting feathering large can still produce a broader, softer transition that follows image features more intelligently. Moving the dotted line farther out only increases the plain gradient width; it won’t try to detect or follow object boundaries.

A practical use is drawing a rough mask around something like a tree, then using feathering to better separate it from the background without tracing every edge precisely.