What is the role of AllowMultiple=true/false?

What is the role of AllowMultiple when we already inherit from SelectOneAttribute?

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]publicclass LanguageSelectionAttribute : SelectOneAttribute
{publicoverride Type SelectionFactoryType
    {
        get
        {returntypeof(LanguageSelectionFactory);
        }
        set
        {base.SelectionFactoryType = value;
        }
    }
}