The company hired a policeman to 'stand sentry' [=be a guard] by the door.
The general placed/posted an armed 'sentry' at the bridge.
(..) something that you do well. Sir, yes, sir. What's your siXth general order?. Sir, the siXth general order is to receive and obey... ...and pass on to the sentry who relieves me... ...all orders.... Sir, the private's siXth general.... The private has been instructed but does not know. (..)1987 Full Metal Jacket