https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.label.associatedcontrolid?view=netframework-4.8.1&redirectedfrom=MSDN
Label.AssociatedControlID Property (System.Web.UI.WebControls) | Microsoft Learn
Gets or sets the identifier for a server control that the Label control is associated with.
property systemweb uilabelmicrosoftlearn