Yes, that is correct.
An example of when OpenACS's subsite functionality can be useful would be as follows:
You are an international hotel chain with a corporate main website and a dedicated website for every hotel you own. Some of these use different languages, some use different character sets and all have their own sets of registered customers.
Any customer can potentially be a guest at any hotel, and whilst some employees may only need access rights within their own hotel website, many of the country and worldwide managers need to be able to access the website of several hotels or even all of them. The MD of the whole organisation needs access to everything.
In this situation, there are a number of ways that openacs can facilitate. Here is one:
You set up a domain name for your hotels group (let's say 'internotels.com'), and you set that up to resolve to your OpenACS 'Main' subsite. This becomes your central corporate website with shareholder info etc.
Because your hotels have fully delegated marketing, you now set up an individual subsite for each hotel, and delegate the admin rights to the individual employee at each hotel who is responsible for it.
The hotels can now operated independently, and you have centralised overall control (technical updates, new capabilities, admin, security, data, intellectual property, central marketing and access/control of customer database and lists).
Each of the subsites can be run as independently or non-independently as you wish. They can each have their own domain names, or operate as a url within the internotels.com
domain. Alternatively you could set them up to be resolved as onehotel.internotels.com, otherhotel.internotels.com
OpenACS is about flexibility and integration.
I hope this simplified example will help.
Have a look at the worldwide Greepeace website for an example.