什么是ASP.NET迁移中的“鉴别器”列？

我需要在ASP.NET MVC 5中的角色标识表中添加一个额外的字段。

我使用迁移。

我为角色添加了扩展：

public class ApplicationRole : IdentityRole
{
    public ApplicationRole() : base() { }

    public ApplicationRole(string name)
        : base(name)
    {
    }

    public virtual Project Project { get; set; }
}   

我正在上的迁移课程是：

public partial class ProjectToIdentity : DbMigration
{
    public override void Up()
    {
        CreateTable(
            "dbo.Projects",
            c => new
                {
                    ID = c.Int(nullable: false, identity: true),
                    ProjectName = c.String(maxLength: 100),
                })
            .PrimaryKey(t => t.ID);

        AddColumn("dbo.AspNetRoles", "Discriminator", c => c.String(nullable: false, maxLength: 128));
        AddColumn("dbo.AspNetRoles", "Project_ID", c => c.Int());
        CreateIndex("dbo.AspNetRoles", "Project_ID");
        AddForeignKey("dbo.AspNetRoles", "Project_ID", "dbo.Projects", "ID");
    }

    public override void Down()
    {
        DropForeignKey("dbo.AspNetRoles", "Project_ID", "dbo.Projects");
        DropIndex("dbo.AspNetRoles", new[] { "Project_ID" });
        DropColumn("dbo.AspNetRoles", "Project_ID");
        DropColumn("dbo.AspNetRoles", "Discriminator");
        DropTable("dbo.Projects");
    }
}

问题是-什么是“鉴别器”列？我的模型中没有这样的专栏。迁移工具为何要添加此字段，其目的是什么？