Airfields are typically categorised for difficulty. Cat A is for airfields where there are few threats. B for airfields which have threats and difficulties which make them more challenging and Cat C is for airfields with significant threats and difficulties. A good example of a Cat C airfiled is Kathmandhu with a very steep VOR approach due to terrrain.
Most airlines stipulate the Captain must be the operating pilot at Cat C airfields, and as a Captain myself - I would feels very wary about trusting someone else when it's my nuts on the chopping block!