categories_as_tabs:

A list of the categories that are displayed as tabs in the user interface. Categories not listed here can still be searched with the Search syntax.

categories_as_tabs:
  general:
  images:
  videos:
  news:
  map:
  music:
  it:
  science:
  files:
  social media:

Engines are added to categories: (compare categories), the categories listed in categories_as_tabs are shown as tabs in the UI. If there are no active engines in a category, the tab is not displayed (e.g. if a user disables all engines in a category).

On the preferences page (/preferences) – under engines – there is an additional tab, called other. In this tab are all engines listed that are not in one of the UI tabs (not included in categories_as_tabs).