Given a class like this:

public class Student
{
public ObjectId Id

{ get; set; }

public List<int> Scores { get; set; }

}

Scores is not persisted as a bson array, but rather as a document.

I would assume this pertains to any property that is not defined as an array.

IEnumerable<T>
ICollection<T>
IList<T>
HashSet<T>
List<T>
etc...