(co-author of the article) I wrote a longer list of categories [1] a while ago (some of which aren't mentioned in the article), here's the list:
* Basic information about countries e.g. population
* Ingredients and dishes from restaurant menus I didn't know
* Important people and places
* History facts (typically from Kindle highlights)
* Conversions between units (e.g. lbs to kg)
* Season for various vegetables and fruits
* Keyboard shortcuts for vim, readline, etc.
* Learning words and terms I don't know from Kindle/Instapaper highlights
* Useful statistics
