Paint Op presets store the preview thumbnail, brush-engine, the parameters, the brush tip, and, if possible, the texture. They are saved as .kpp files.
For information regarding the brush system, see Brushes.
The docker for Paint-op presets is the プリセットドッカー. Here you can tag, add, remove and search paint op presets.
Editing the preview thumbnail¶
You can edit the preview thumbnail in the brush-scratchpad, but you can also open the *.kpp file in Krita to get a 200x200 file to edit to your wishes.