Plugins add tabs to the navigation when on_project_tabs event is trggered. Every module (plugin) has a on_project_tabs handler located in /activecollab/application/modules/#MODULE_NAME#/handlers/on_project_tabs.php file. Here is how handler usually looks like:

function calendar_handle_on_project_tabs(&$tabs, &$project) {
  $tabs->add('calendar', array(
    'text' => lang('Calendar'),
    'url' => Calendar::getProjectCalendarUrl($project),
  ));
} // calendar_handle_on_project_tabs

If you wish to hide a certain type from your clients alter it too look like this:

function calendar_handle_on_project_tabs(&$tabs, &$project) {
  $user = get_logged_user();
  if(instance_of($user, 'User') && $user->isOwner()) {
    $tabs->add('calendar', array(
      'text' => lang('Calendar'),
      'url' => Calendar::getProjectCalendarUrl($project),
    ));
  } // if
} // calendar_handle_on_project_tabs

Notice that we are asking from the system to tell us who the logged in user is and if we have a logged in user and he is a member of owner company that we are adding a new tab.

Tabs are added by the following modules:

- Discussions - /discussions
- Milestones - /milestones
- Files - /files
- Checklists - /checklists
- Calendar - /calendar
- Pages - /pages
- Tickets - /tickets
- Time - /timetracking

No - permissions and client visibility of aC functions are quite broken in the current release. All users on a project have view permission to everything. Even if you hide a tab, that content still appears on the dashboard, on the calendar, in linked modules, etc. See this post for details:

http://www.activecollab.com/forums/topic/2327/

yes indeed... so it would be a lot easier to explain to clients or new contractors how we work on AC