Page 42 - Beginner's Guide to Aviation

International airports can either be a primary or secondary hub. Primary hubs are the main base for several airlines, usually national carriers. For instance, Singapore Airlines is our national carrier and its primary hub is Changi Airport. Primary hubs are usually BIGGER than secondary hubs because they serve a large number of transfer passengers. Secondary hubs are smaller and serve lesser connections than a primary hub.
Now let’s move on to Domestic/Regional Airports. International Airports and Domestic Airports can be distinguished easily by one interesting fact. International Airports require Customs and Immigration, but Domestic Airports do not. Therefore, you do not need to bring your passport to travel at Domestic Airports!
REPUBLIC OF SINGAPORE
PASSPORT