https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.design.designerregion.designerregionattributename?view=netframework-4.8.1
DesignerRegion.DesignerRegionAttributeName Field (System.Web.UI.Design) | Microsoft Learn
Defines the HTML attribute name for a designer region.
web ui designfield systemmicrosoftlearn