Global administrators can assign users to a different posts and with different permissions. For example, you can be a regular user in post AIT Taipei, but also you can be administrator in Buenos Aires. This is happening when you press the Users button at the sidebar and click on the right button to the user's name.
Press the Details button you see all details about the selected user and the post roles that the selected user is having. You must click the Add Role button and you will be redirected to another view with all posts and option to make the selected user administrator for the selected post and after you click Create button, selected user having permissions to see the tasks of the selected post.