It’s eg. in https://eu.store.bambulab.com/products/bambu-cool-plate
“Before auto-leveling, it is necessary to repeatedly rub the nozzle in the special wiping area of the build plate to completely remove any residual material at the tip of the nozzle. The coating in the special-designed wiping area will gradually become worn over time. This is normal and does not affect print quality or nozzle lifetime, so there is no need to worry about any quality issues”