Multiple sites off one installation
A Datafeed Studio license entitles you to run a single Datafeed Studio installation site (i.e. administration interface).
From this single installation you can run up to 10 DS powered sites (depending on your license type) typically by using domain aliasing (see http://webwizardworks.com/web-design/domain-alias.html) or, if you are adept at using the UNIX command-line, symlinking.
The basic premise of multiple sites is you have one master site, let's call it master.com, which acts as the central server for your sites, and any other sites you have, share the same DS files and database as this, i.e. you don't have to install DS and a new DB for each one.
Domain Aliasing solution
Most hosting control panels offer a solution for adding domain aliases to an existing domain. My advice is to do a Google Search for +your_host +"domain alias" as it differs on a per host basis.
Once you have added an alias to your main DS domain, e.g. if your main DS site is master.com, you might have an alias named slave.com, you can then register the site at the DS admin interface and add a store to it that will operate independently from other store(s) you may have running from your existing installation.
Advanced UNIX solution
It is a very UNIX/command-line based solution so it might take a while to wrap your head around it.
OK, let's imagine you have DS installed in master.com, so if you had ssh access to your host and you issue a list command (ls) you should see :
as a folder.
Now, on the hosting I use, if I install another domain, let's call it slave.com, my host will create another folder at the same level, e.g. :
in which I can stick the usual HTML files for my sites. So of I do a listing now I would see :
and both are folders.
The trick is that we don't want slave.com to be a folder in it's own right, we want any requests to HTML files for that to look like they are going to slave.com, but actually access the files in master.com.
In UNIX you can use what is called a symbolic link to achieve this (think of it as a little bit like a Windows shortcut link if that environment is more familiar).
To do this we first need to remove the folder version of slave.com like so :
$ rm -rf slave.com
Next we need to link it to the master folder :
$ ln -s master.com slave.com
Now when you do a directory listing, it should look like :
slave.com -> master.com
The arrow sign indicates it is linked to master.com.
Now using the master.com DS admin panel, you can create a new site called slave.com, so in the DS registered sites you would see both master.com and slave.com.
Now, if you point your browser to http://slave.com you should see a DS site that can uses the same products as the ones defined by master.com.
At least, that's the theory :-p
No, seriously I use this technique for my sites, I have on master database of all my feeds, and then I create smaller sites that use a subset of the data as I usually define the categories myself - i.e. I don't just show all the imported categories.