(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

[1]: https://news.ycombinator.com/item?id=17847031