Using key property in Web Controls

[vladibo]

I have noticed the usage of key property in web controls but I don't see it in the documentation. For example I see this code:

<asp:Label id="SystemLabel" runat="server" key="Main_SystemLabel"></asp:Label>

There is no property key in the Label control, nor in one of the base classes: System.Web.UI.Control, System.Web.UI.WebControls.WebControl

What does it mean? Is there a way to access it by something like Page.FildByKey("Key")