If you need to secure 3-fields domains such as:
support.yourdomain.com
buy.yourdomain.com
sell.yourdomain.com
submit.yourdomain.com
Then you should use *.yourdomain.com for your CSR's Common Name.
If you intend to secure 4-fields domains such as:
leve1.support.yourdomain.com
dealer.buy.yourdomain.com
dealer.sell.yourdomain.com
Then you should use *.*.yourdomain.com for your CSR's Common Name.
Wildcards can also have more fields, such as:
*. * . * . yourdomain.com
* . * . * . * . *. yourdomain.com etc.
But, if you get a 5-field wildcard, then it only works for 5-field web sites.
If you are running IIS 5.0, you cannot get a multi-asterisk wildcard but you can get a single asterisk wildcard, such as *.yourdomain.com.
Download the CSR Generation Guide for IIS 6- Windows Server 2003.