Each Channel object will create an Active Scripting Host to run a specified agent. When doing so, it passes dispinterfaces to the host that the agent script can then access and modify in its own namespace. Thus, these interfaces appear as "built-in" interfaces to the agent.
Digression: Other Examples of Active Scripting Hosts
The ActiveX Scripting Host technology is perhaps unfamiliar, at least in name, to many developers. To get a more concrete idea of exactly how this system works, a few common examples of ActiveX Scripting Hosts are listed below.
ASP
A well known Active Scripting Host is the Active Server Pages (ASP) system that Internet Information Server (IIS) 3.0 and 4.0 support. In this case, the IIS server instance creates an Active Scripting Host and passes the Request, Response, Server, Session, and Application dispinterfaces to the host. The ASP script can then access these objects via these interfaces in the script without having to explicitly create them. The script to parse is the one identified by the client request usually with the extension ".asp"
Windows Scripting Host
This host is a newcomer, but the principle is the same. When a user types cscript.exe <filename>.<ext> the executable generates an Active Scripting Host and passes the host the WScript dispinterface. Thus WSH scripts can use the built-in WScript object directly.
For more information on the Active Scripting Host system, visit the Microsoft Developer's Network at http://msdn.microsoft.com.