This allows you to retrieve the selection set of fields that have been asked for when the
DataFetcher
was invoked.
For example imagine we are fetching the field 'user' in the following query
{
user {
name
age
weight
}
}
The selection set in the case above consists of the fields "name, age and weight".
You can use this selection set perhaps to "peek" ahead and decide that field values you might need
from the underlying data system. Imagine a SQL system where this might represent the SQL 'projection'
of columns say.