While you don’t need to understand how data is stored to use Multisite for iWeb it can be helpful occasionally to understand what is going on behind the scenes. At a nuts and bolts level Multisite for iWeb stores all of its sites in your home folder’s Documents folder in a single package (which is really a fancy folder dressed up to look like a file) called “Multisite for iWeb Data.mfi.”
Inside the package there is a folder called “sites” and within that is a folder with the name of each site you have managed. In each of those folders is a single “Domain” file that iWeb uses when it’s editing that site. Often a picture is easier to understand. This snapshot is a look inside of a package that holds four separate sites.