Like dktc said, the passengers of the airport is the indicator, but it sometimes varies.
For example:
Your home base is A. It's gate value is 50,000 euros per gate. If you fly to Airport B, that has a gate value of 50,000 euros per gate, then you will get a poor profit, sometimes, even a loss.
BUT if A's gate value is 200-300,000 euros per gate, and you fly to B, which is still 50,000 euros per gate, you will get a higher profit, and it will probably be rated as medium.
This is from my experience (2 years+ experience, that is), so please correct me if I made a mistake