Understanding site collections

To help you develop and maintain your production websites, you can create site collections, which bundle and organize related websites.

A site collection contains

a primary website (to which you can associate a custom domain name), and
one or more secondary websites

Site visitors can view the primary website in a site collection using one of the following URLs:

The individual website's Acquia Cloud Site Factory URL
The site collection's URL
One of the custom domain URLs associated with the site collection

For example, you have a production website for your department, but you also have a staging website that contains new content changes you're currently developing. Having both of these websites in a site collection, you can associate the current production website with a custom domain URL.

When the staging website is complete, you can swap the websites, making the staging website the production website with the custom domain URL (without having to contact the campus host clerk regarding the website change).

Notes

Only user accounts with the site administrator or site builder role can create or manage site collections.
The websites contained by a site collection are completely separate from one another and do not have any common workflow capabilities (development, staging, production). Updates made to one site are not shared with the others.
You can duplicate a production website to create a secondary development website in the site collection, and then later promote it to the primary website. (Note that the new primary website doesn't contain databases or user-created items created after you duplicated the website.)
If your site is already live when you decide to create a site collection it is vital to note that while you will not see any interruption in your site's availability on the web, you MUST take note of the new Site Factory domain name generated at the time of your site collection and update the campus host clerk to request a new CNAME assignment for your domain name.
If you're duplicating your existing site in order to create a sandbox or safe experimentation space, a site collection is not needed. This process is only intended if the original site is going to be swapped out for a replacement.