Begging your pardon, but his here Wiki should probably be updated with information about specialist function. I found "UseSpecialistBonus = True" lines all over my config and don't know what to make of it.

If this is set to true. Which they all are. Then having an Engineer (converter trait) will apply an efficiency multiplier to the converter based on their star skill level. of 0.5 for each level.
This is the same way the ISRU converters work in the stock game.
It's in the changelog since v0.12.9:
Changed Converter parts to have a bonus base of 1 and specialist efficiency factor of 0.5. This should make the values more what they should be. (Start at 100%).
I agree - the descriptions could be updated.. but then I would have to ask everyone to volunteer to update the localization texts for the other languages.
There is also a debug option in the stock game that will show you the efficiency effect for them in their PAW if you turn it on (second option here):