请教 MongoDB 驱动中的一段继承代码含义

using MongoDB.Driver;
using System;

/// <summary>
/// The result of a delete operation.
/// </summary>
public abstract class DeleteResult
{
	/// <summary>
	/// The result of an acknowledged delete operation.
	/// </summary>
	public class Acknowledged : DeleteResult
	{
		private readonly long _deletedCount;

		/// <inheritdoc />
		public override long DeletedCount => _deletedCount;

		/// <inheritdoc />
		public override bool IsAcknowledged => true;

		/// <summary>
		/// Initializes a new instance of the <see cref="T:MongoDB.Driver.DeleteResult.Acknowledged" /> class.
		/// </summary>
		/// <param name="deletedCount">The deleted count.</param>
		public Acknowledged (long deletedCount);
	}

	/// <summary>
	/// The result of an unacknowledged delete operation.
	/// </summary>
	public class Unacknowledged : DeleteResult
	{
		private static Unacknowledged __instance = new Unacknowledged ();

		/// <summary>
		/// Gets the instance.
		/// </summary>
		public static Unacknowledged Instance => __instance;

		/// <inheritdoc />
		public override long DeletedCount {
			get;
		}

		/// <inheritdoc />
		public override bool IsAcknowledged => false;

		private Unacknowledged ();
	}

	/// <summary>
	/// Gets the deleted count. If IsAcknowledged is false, this will throw an exception.
	/// </summary>
	public abstract long DeletedCount {
		get;
	}

	/// <summary>
	/// Gets a value indicating whether the result is acknowledged.
	/// </summary>
	public abstract bool IsAcknowledged {
		get;
	}

	internal static DeleteResult FromCore (BulkWriteResult result);
}

上面的 Acknowleged 继承于 DeleteResult 可以是为啥要写到 DeleteResult 之中？岂不是造成循环了？