What is the point of the key parameter in ModelState.AddModelError in ASP.NET MVC?

I've added validation checks in my controller that modify the ModelState if the validation fails.

For example:

private bool ValidateMoney(string raw, string name, decimal min, decimal max) {
    try {
        var dec = Convert.ToDecimal(raw);

        if (dec < min) {
            throw new ArgumentOutOfRangeException(name + " must be >= " + min);
        }
        else if (dec > max) {
            throw new ArgumentOutOfRangeException(name + " must be <= " + max);
        }
    }
    catch (Exception ex) {
        ModelState.AddModelError(name, ex.GetUserMessage());
    }
    return ModelState.IsValid;
}

However, I never know value to pass for the key parameter in ModelState.AddModelError. (In the example, I just set it to my UI display name.)

What is the parameter for and how should I use it?