Use case of nesting an enum into a struct
In my opinion nothing indicate the use of enum.
According to my understanding your object provides the following functionalities from consumer perspective:

Provide two factory methods one for Ascending and another for Descending
Provide custom string representation (abbreviation) for these

From the public interface point of view the following implementation is equivalent with yours
public struct OrderDirection 
{
    private readonly string _direction;
    private OrderDirection(string direction)
    {
        _direction = direction;
    }
      
    public override string ToString() =&gt; _direction;
    
    public static OrderDirection Ascending = new OrderDirection(&quot;asc&quot;);
    public static OrderDirection Descending = new OrderDirection(&quot;desc&quot;);
}