What fields does getSelectFields() actually select and why?

[drewg2009]

From the docs

Eager loading relationships
The Rebing\GraphQL\Support\SelectFields class allows to eager load related Eloquent models. Only the required fields will be queried from the database.

The class can be instantiated by typehinting SelectFields $selectField in your resolve method.

You can also construct the class by typehinting a Closure. The Closure accepts an optional parameter for the depth of the query to analyse.

How do I specify the the required fields?

Only the required fields will be queried from the database.

How does it know which fields to select?