ASP.NET迁移中的Discriminator列是什么？

我使用迁移.

我添加了角色的扩展名,如：

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

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

    public virtual Project Project { get; set; }
}

我的迁移课程是：

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","dbo.Projects","ID");
    }

    public override void Down()
    {
        DropForeignKey("dbo.AspNetRoles","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Discriminator专栏？我的模型中没有这样的专栏.为什么迁移工具会添加此字段以及它具有哪些目标？