There are a lot of questions. This forum will get you there, though.
Here's my take on subdomains...
It's cool to buy the package with unlimited subdomains. It's great to have the ability to play with lots of sites that hang off your primary domain name.
'Unlimited Subdomains' are based on your primary site's URL. You got that one already, so good for you. It took me a while to get that one.
When you create a new site (a site with a completely different URL otherwise known as a domain name), this site is still technically considered sub domain according to your host. However, they are totally different sites being they have a entirely separate domain names. A little confusing, I know.
All in all, it's good to have the free unlimited subdomain feature because (and this is just a couple examples as to why) you can create a bunch of sites and have the ability to create additional landing pages for products. You can create a landing page within your primary domain, but sometimes people like to keep the sales page separate. Different strokes for different folks.
But try not to get it twisted...when you buy a new domain name, it's really a subdomain registered under your primary account...which is typically tagged as your first website's domain name.
And if you're curious, each time you register a new domain name, you pay for that domain name. It ain't free.
- You buy a hosting package and title your site's name: mysite.com
- You decide to create a landing page separate from your site but tied to your site's domain. You title it : widgetsforsale.mysite.com
- This is a free subdomain you tack onto your primary website's address.
Now, if you want to register a totally different site such as beachbody.com, you now have a completely different site because you're purchasing a new domain name. However, and I'm sure someone will correct me if my wording isn't exact, this site is a still subdomain in the eyes of your host because it's registered under your primary account which is in this example: mysite.com
Since I store my site's files in my publichtml directory, I can see all of my sites listed in my primary site's publichtml directory along with my free subdomains.
In other words, within my publichtml files my sites are either labeled with different URL's or id'd as my primary domain name prefixed with whatever I choose as the prefix.
Hope this helped you a little. I just felt like trying to tackle this question since you have't received a response yet.