https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.persistencemodeattribute.attribute?view=netframework-4.8.1
PersistenceModeAttribute.Attribute Field (System.Web.UI) | Microsoft Learn
Specifies that the property or event persists in the opening tag of the server control as an attribute. This field is read-only.
field systemweb uiattributemicrosoftlearn