A Multi-Hosted domain is like an entirely separate site in your own account. It can have its own domain, its own FTP login details, and is totally separately hosted.
A subdomain, on the other hand, appears as companyname.yourdomain.com and then forwards the user (invisibly perhaps) to
http://www.yourdomain.com/whatever/s...ory/you/want/.
So if you are wanting to set up example sites with their own domains, yes - you want Multi-Hosted domains, of which a NexBusiness account has 2, and can have (if paid for) up to 10.
After that point you would want to look at the options for resellers, who I believe can set up unlimited domains in this manner - but of course you still have to pay for the domains themselves ($12.95 from HN).
If it would be fine for sample sites to be samplecompany.yourdomain.com, then you've got 10 subdomains to play with on NexBusiness.