You use this when you want to use a user control in your aspx or ascx page declaratively. @Register
creates an association between a tag prefix and a custom control, which provides developers with a concise way to refer to custom controls in an ASP.NET application file (including Web pages, user controls, and master pages).
<%@ Register tagprefix="tagprefix"
namespace="namespace"
assembly="assembly" %>
<%@ Register tagprefix="tagprefix"
namespace="namespace" %>
<%@ Register tagprefix="tagprefix"
tagname="tagname"
src="pathname" %>
assembly: The assembly in which the namespace associated with the tagprefix attribute resides.
namespace: The namespace of the custom control that is being registered.
src: The location (relative or absolute) of the declarative ASP.NET User Controls file to associate with the tagprefix:tagname pair.
tagname: An arbitrary alias to associate with a class. This attribute is only used for user controls.
tagprefix: An arbitrary alias that provides a shorthand reference to the namespace of the markup being used in the file that contains the directive.
