You can get compartment syndrome on a few areas, but it's most common in calf just because of the structure of the lower leg. In comparison, the thigh and upper arm are much "roomier" so it's not likely to happen there. You can also get abdominal compartment syndrome.
It is just when there is too much swelling in one of the compartments and the swelling is leading to impaired blood circulation (which then kills off nerves who can't get a good blood supply).
So it's always a risk after lower limb surgery in particular, your leg is usually immobilised, maybe in a cast, and the natural process of inflammation causes fluid to increase in the tissues. So it's important for Nurses to do regular neurovascular observations to check you have normal movement, sensation, pulses, and your pain is controlled.
The classic way to remember a compromised limb is: "pale, pulseless, paraesthesia (change in sensation), paralysis, perishingly cold (to touch)".
However if you have all those you're pretty fucked they are quite late signs.