Assembly: System.Windows (in System.Windows.dll)
The value is settable on control instances through the AutomationProperties.HelpText attached property.Notes to Inheritors
For accessibility purposes, HelpText is often used to communicate instructions to the user, beyond the simple descriptive label that is provided by the UI Automation Name and possibly by the localized control type name for example "Button". In some controls, the HelpText is used to communicate control state.
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.