Hi there!
The docs for the IPublishedContentQuery - such as IPublishedContentQuery | Umbraco CMS - say:
By default,
IPublishedContentQuerywill search on Umbraco’s ‘External’ search index
However, I couldn’t find any reference to what this actually means in the docs themselves. Could someone please link me to the details of Umbraco’s ‘External’ search index?
TIA!
Found some references in the Examine “Quick-start” but it feels strange to not have a separate page for the three indexes listed. Guess someone might have to propose that… 
External index is the published content and media, which is why it is by far the most used one as that is normally what you want to search in on the website.
The internal index also contains unpublished nodes, and is the one that powers the backoffice search for example.
Member, delivery api, forms are all pretty self-explanatory I think 