After you've installed and mounted your package, you can configure each instance to act as you would like.
This is done from the Applications page. Log in, go to the Admin or Control Panel, click on the subsite the application is in, and click on Applications. If you click on the 'Permissions' link, you will see and be able to set the permissions for that application.
Each application may have different behavior for what Read Create Write and Admin permissions mean, but generally the permissions are straightforward. If you find the behavior is not what you expect after setting permissions, you can post a bug in the OpenACS bugtracker.
'The Public' refers to users to the website who are not logged in. 'Registered Users' are people who have registered for the site.