Hi,

For folder based web sites there is only 1 domain and it can only have 1 SSL certificate because SSL certificates are tied to a specific domain. So you can use the same SSL certificate for all sites in a multi sites installation based on folders. 

When you have multiple sites based on host names, then you run into a problem because only 1 SSL certificate can bind to a particular ip address on port 443 (the SSL port). Therefore you have to make a separate IIS web site in this case and use a different ip address, however it can still point to the same physical installation folder as the first site.

Hope it helps,

Joe