To classify an enclosure or area, it is necessary to identify how freely mobs can move in and out of the enclosure and also if mobs can appear in the enclosure. Usually, the barrier keeps out while the interior tries to make sure no mobs can spawn inside.

Barrier

A-Class Barrier

Keeps out all mobs. Enderdragon and explosion proof. Inflammable and indestructable. Skelatons cannot shoot arrow over.

B-Class Barrier

Keeps out all land mobs. Creepers and skelations not able to do damage. Keeps out air-mobs but not explosions.

C-Class Barrier

Keeps out all ground mobs, spiders included.

D-Class Barrier

Keeps out all mobs but spiders and spider-jockeys.

E-Class Barrier

Keeps ground mobs, except spiders, out most of time, but they can slip through gaps by climbing and manuvering. Very large passive barriers, like 30+ meters of water, also are in this classification.

F-Class Barrier

Passive barrier, like a hill.

G-Class Barrier

No barrier.

Interior

Type-I

Completely spawnproof. Ghasts and airmobs cannot spawn anywhere, nor can slimes. Caves capped.

Type-II

Spawnproof for all ground mobs. Caves capped.

Type-III

Mostly spawnproof except inaccesible corners. 99%+ spawnproof

Type-IV

Semi-spawnproof except for cliffs and hard to reach areas. 90% spawnproof.

Type-V

Mob spawning passively decreased by use of mob-sponges such as mob spawning rooms.

Type-VI

No modifications to terrain. Natural spawning.

Type-VII

Mob spawns artificially increased.