A detection type is how the enemy will notice the players. You can set different types or all together.
Each detection type has the Detection Sensitivity value - it means how quickly opponents will notice the characters.
The vision is divided into 2 parts - central vision and peripheral. During the game, if a player is in sight, the enemy will see him. And depending on the AI behavior, the enemy will immediately attack the player or start looking for him.
Here you'll find more info about the AI behavior:
All characters have a noise radius, if the enemy is in that area and there are no walls between them, then he hears the character.
If a player comes close to the enemy, he detects him.