Sure. The Business Plan offers 10 unique domains under 1 account. Each of those domains could also be a subdomain of your primary account's domain. In your public_html folder, you will have a specific folder for each domain hosted under your account. So, for example, you have 3 clients, each of which has their own domain, such as client1.com, client2.com, and client3.com. Their websites could be accessed in 3 different ways.
1) client1.com
2) client1.yourdomain.com
3) yourdomain.com/client1
(The same goes for client2 and client3)
How you choose to access the sites is a matter of personal preference. The one thing that I will mention is that this gives you the ability to configure the site before making it live on the customers domain. So you could access it and make changes by going to client1.yourdomain.com, and when ready, point client1.com to your account so that the rest of the world can see the website.
I hope that helps to answer your question. Since you have a specific business need, I will make both Hostable Domains and Subdomains unlimited if you choose to use our services. This will give you the most flexibility to meet the needs of your clients while keeping costs low.