The documentation pages of some brush engines sometimes contain jarringly uncertain (and perhaps unprofessional) explanations. Documentation is supposed to explain features with certainty.

https://docs.krita.org/en/reference_manual/brushes/brush_engines/curve_engine.html

"Smoothing, which… I have no idea actually. I don’t see any differences with or without it. Maybe it’s for tablets?"

https://docs.krita.org/en/reference_manual/brushes/brush_engines/hatching_brush_engine.html

"When I first tried this brush, my impression of it was “plain parallel lines” (and the award for most boring brush goes to…). Fortunately, existing presets gave me an idea of the possibilities of this brush."

"Before going on: at the time of this writing, there is a bug that causes line thickness to not vary on default settings. To get around this, go to Hatching preferences and check Antialiased Lines. Pentalis is aware of this issue so the bug may get fixed soon."

"I’m guessing this affects the rendering quality, but you won’t see much of a difference. Check this if you want to."

Somewhat related:
https://docs.krita.org/en/reference_manual/brushes/brush_engines/bristle_engine.html

"Be aware that this feature is a bit buggy though. It’s supposed to take the color from the current layer, but some buggy behavior causes it to often use the last layer you’ve painted on (with a non-Bristle brush?) as source. To avoid these weird behaviors, stick to just one layer, or paint something on the current active layer first with another brush (such as a Pixel brush)." (Is this still an issue or is it at least reported?)