Sub Businesses are small businesses that are run by your airline that are build able on any airport that you serve. There are three types of sub-businesses.
This is the cheapest subsidiary business. It makes a profit of around 600-700k.
This is the second subsidiary business available in-game. It makes 1,500k+ profit.
This is the most expensive subsidiary business. Although it is expensive, it gives a return of 5,000k to 14,000k.