On Microsoft's IIS web server, what Apache (and everyone else) calls virtual hosts are called sites. A single site can listen on multiple hostnames and ports, with and without SSL. Each web site is assigned a number by IIS called its INSTANCE_ID. This is just a number that is used within the IIS MetaBase to refer to the site.
It used to be generated sequentially as you created new sites, but as of new versions, the number is generated by some kind of algorithm based on the site name, and tends to be a large number. In these versions, the IIS GUI tool will display the site ID in the site list. If it does not, you can probably assume that each site is numbered starting from 1 reading down the list.
To find out the number, you can also place a simple ASP script on the site and use it to display the Request("INSTANCE_ID") value.
The Shibboleth SP currently requires that you tell it what the proper fully-qualified hostname for each web site is by using <Site> elements in the ShibbolethXML file. Each site that the filter is expected to pay attention to must be entered. A site that is not listed will be ignored by the filter.
-- Main.ScottCantor - 03 Mar 2005