When a user makes a search query, SharePoint doesn’t go straight to the Index.
Instead the query passes through a Result Source.
There are 16 built-in Result Sources.
The default is ‘Local SharePoint Results’, which is all SharePoint content, except People results.
Conversely, there’s one called ‘Local People Results’, which returns only User Profile-based search results.
There are others, for specific slices through the Index.
For instance: Conversations, Documents, Pictures, Wiki, and many others.
You can also make your own.
You typically build a custom Result Source to return either more or fewer results.
A Result Source contains a Query Transform – a Keyword Query statement that tells SharePoint how to augment their query.
For instance – if they searched for “HR”, only include results from the Human Resources Site Collection.
You can also use Result Sources to widen the search scope.
Make the transform blank, and SharePoint will search through all the available Content Sources.