What is a SUB domain ?
A sub domain:
You create a subdomain using your domain.
www.shop.rdcss.com - if you do this the www.shop. must alway be there or it will not work.
rdcss.com - is the domain name
shop - is a subdomain
www.shop. - is a subdomain name
note the subdomain namd is alway attached to the main domain.
There is no charge for subdomains off your domain, you can have as many as you need.