PlaceHolder

Namespace:

System.Web.UI.WebControls

Assembly:

System.Web.dll

https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.placeholder?view=netframework-4.7

Using Panel or PlaceHolder

https://stackoverflow.com/questions/483352/using-panel-or-placeholder

A panel expands to a span (or a div), with it's content within it. A placeholder is just that, a placeholder that's replaced by whatever you put in it.