Can you clarify this from the installation guide:
Default Blog Domain: If the user does not enter a domain, this will be used by default. If you're not using Custom fields (see below) this is the default domain name that will be used with numbers appended (to keep it unique). This cannot be changed later without you editing your database directly
Not sure what it means about numbers appended, can you clarify?
Also, if we want to use domain mapping, does the mapping have to be done at order time via WHMCS, or can users map domains to their sites later by pointing to the IP of the multisite install and adding the domain in the Domain Mapping settings?