实体多对多关系，Entityframework自动生成的数据库中Role_RoleID有什么用？

实体类定义如下：

public class User
{
[Key]
public int UserID { get; set; }

/// <summary>
/// 用户名
/// </summary>
[Required(ErrorMessage = "必填")]
[StringLength(20, MinimumLength = 4, ErrorMessage = "{1}到{0}个字符")]
[Display(Name = "用户名")]
public string UserName { get; set; }

……

public virtual ICollection<UserRoleRelation> UserRoleRelations { get; set; }
}

public class Role
{
[Key]
public int RoleID { get; set; }

[Required(ErrorMessage ="必填")]
[StringLength(20,MinimumLength =2,ErrorMessage ="{1}到{0}个字")]
[Display(Name ="名称")]
public string Name { get; set; }

……
public virtual ICollection<UserRoleRelation> UserRoleRelations { get; set; }
}

public class UserRoleRelation
{
[Key]
public int RelationID { get; set; }

/// <summary>
/// 用户ID
/// </summary>
[Required()]
public int UserID { get; set; }

/// <summary>
/// 角色ID
/// </summary>
[Required()]
public int RoelID { get; set; }

}

生成的数据库结构如下，请问Role_RoleID有什么用？