Aggregation

It's a relationship where child can exist independently of the parent.

public class Invoice
{
   public Guid Id { get;set; }

   public DateTime DateTime { get; set; }

   [Aggregation]
   public InvoiceType Type { get; set; }
}

public class InvoiceType
{
   public Guid Id { get; set; }

   public string Name { get; set; }
}