Do I need to purchase an additional domain seat to use RB2B on a subdomain of a domain I'm already using?
No — you do not need to purchase an additional domain seat to use RB2B on a subdomain of a domain that's already active.
If your domain is entered as a naked domain (for example, domain.com without "www" or any other subdomain), RB2B will automatically function on all subdomains of that domain where the RB2B script is installed.
Will I be billed for an unfilled seat at the end of my billing cycle?
At the start of a new billing cycle (monthly or yearly, depending on your plan), any unused seats — those not assigned to a domain — are removed from your account, and you won't be charged for them in the renewal.