In nightmare and hell mode, the Area Level is basically used to determine the types of items (and their mods) that can be dropped from monsters, as well as changing to-hit calculations on a monster. It also also plays a role in determining the possible item types from chests and chest-like objects, in all difficulty levels.
For a useful Diablo 2 Expansion (1.10+) example, grand charms that appear with a +skillbranch mod can only appear on monsters that have been killed from The Great Marsh in act 3 in nightmare difficulty onwards. Technically, the +skillbranch mod can only appear on items of ilvl 50+, and the ilvl of an item is equal to the mlvl of the monster that dropped it. In normal, mlvl is equal to the monster's level, while in nightmare/hell, mlvl is equal to the area level the monster is found in.
Via [1]:
Ilevel 50+ = plain skillers (plain mosters NM Great Marsh onwards)
ilevel 61+ = skillers with 31-35 life (plain monsters most of Act 5 NM onwards)
ilevel 77+ = skiller with 36-40 life (plain monsters Hell Lost City onwards; Pindle)
ilevel 91+ = skiller with 41-45 life (Hell Baal ilevel 99, Hell Nithy ilevel 95 and Hell Diablo ilevel 94)
Exceptions:
Champion and Unique monsters have monster levels and drop items with item levels of +2 and +3 of their area's level, respectively.
Super Unique monsters have their own special monster levels and drop items with item levels independent of their area's level.
| Act | Normal alvl | Nightmare alvl | Hell alvl | Level Name |
|---|---|---|---|---|
| Act 1 | 1 | 36 | 67 | Blood Moor |
| Act 1 | 2 | 36 | 68 | Cold Plains |
| Act 1 | 4 | 37 | 68 | Stony Field |
| Act 1 | 5 | 38 | 68 | Dark Wood |
| Act 1 | 6 | 38 | 69 | Black Marsh |
| Act 1 | 8 | 39 | 69 | Tamoe Highland |
| Act 1 | 1 | 36 | 79 | Den of Evil |
| Act 1 | 2 | 36 | 77 | The Cave Level 1 |
| Act 1 | 4 | 37 | 69 | Underground Passage Level 1 |
| Act 1 | 5 | 38 | 80 | The Hole Level 1 |
| Act 1 | 7 | 39 | 85 | The Pit Level 1 |
| Act 1 | 2 | 37 | 78 | The Cave Level 2 |
| Act 1 | 4 | 38 | 83 | Underground Passage Level 2 |
| Act 1 | 5 | 39 | 81 | The Hole Level 2 |
| Act 1 | 7 | 40 | 85 | The Pit Level 2 |
| Act 1 | 3 | 36 | 80 | Burial Grounds |
| Act 1 | 3 | 37 | 83 | Crypt |
| Act 1 | 3 | 37 | 85 | Mausoleum |
| Act 1 | 7 | 38 | 75 | Tower Cellar Level 1 |
| Act 1 | 7 | 39 | 76 | Tower Cellar Level 2 |
| Act 1 | 7 | 40 | 77 | Tower Cellar Level 3 |
| Act 1 | 7 | 41 | 78 | Tower Cellar Level 4 |
| Act 1 | 7 | 42 | 79 | Tower Cellar Level 5 |
| Act 1 | 8 | 40 | 70 | Monastery Gate |
| Act 1 | 9 | 40 | 70 | Outer Cloister |
| Act 1 | 9 | 40 | 70 | Barracks |
| Act 1 | 10 | 41 | 71 | Jail Level 1 |
| Act 1 | 10 | 41 | 71 | Jail Level 2 |
| Act 1 | 10 | 41 | 71 | Jail Level 3 |
| Act 1 | 10 | 41 | 72 | Inner Cloister |
| Act 1 | 11 | 42 | 72 | Cathedral |
| Act 1 | 11 | 42 | 72 | Catacombs Level 1 |
| Act 1 | 11 | 42 | 73 | Catacombs Level 2 |
| Act 1 | 12 | 43 | 73 | Catacombs Level 3 |
| Act 1 | 12 | 43 | 73 | Catacombs Level 4 |
| Act 1 | 6 | 39 | 76 | Tristram |
| Act 1 | 28 | 64 | 81 | Secret Cow Level |
| Act 2 | 14 | 43 | 75 | Rocky Waste |
| Act 2 | 15 | 44 | 76 | Dry Hills |
| Act 2 | 16 | 45 | 76 | Far Oasis |
| Act 2 | 17 | 46 | 77 | Lost City |
| Act 2 | 18 | 46 | 77 | Valley of Snakes |
| Act 2 | 16 | 48 | 79 | Canyon of the Magi |
| Act 2 | 13 | 43 | 74 | Sewers Level 1 |
| Act 2 | 13 | 43 | 74 | Sewers Level 2 |
| Act 2 | 14 | 44 | 75 | Sewers Level 3 |
| Act 2 | 13 | 47 | 78 | Harem Level 2 |
| Act 2 | 13 | 47 | 78 | Palace Cellar Level 1 |
| Act 2 | 13 | 47 | 78 | Palace Cellar Level 2 |
| Act 2 | 13 | 48 | 78 | Palace Cellar Level 3 |
| Act 2 | 12 | 44 | 78 | Stony Tomb Level 1 |
| Act 2 | 12 | 44 | 79 | Halls of the Dead Level 1 |
| Act 2 | 13 | 45 | 81 | Halls of the Dead Level 2 |
| Act 2 | 14 | 47 | 82 | Claw Viper Temple Level 1 |
| Act 2 | 12 | 44 | 79 | Stony Tomb Level 2 |
| Act 2 | 13 | 45 | 82 | Halls of the Dead Level 3 |
| Act 2 | 14 | 47 | 83 | Claw Viper Temple Level 2 |
| Act 2 | 17 | 45 | 84 | Maggot Lair Level 1 |
| Act 2 | 17 | 45 | 84 | Maggot Lair Level 2 |
| Act 2 | 17 | 46 | 85 | Maggot Lair Level 3 |
| Act 2 | 17 | 46 | 85 | Ancient Tunnels |
| Act 2 | 17 | 49 | 80 | Tal Rasha's Tomb |
| Act 2 | 17 | 49 | 80 | Tal Rasha's Chamber |
| Act 2 | 14 | 48 | 79 | Arcane Sanctuary |
| Act 3 | 21 | 49 | 79 | Spider Forest |
| Act 3 | 21 | 50 | 80 | Great Marsh |
| Act 3 | 22 | 50 | 80 | Flayer Jungle |
| Act 3 | 22 | 52 | 80 | Lower Kurast |
| Act 3 | 22 | 52 | 81 | Kurast Bazaar |
| Act 3 | 23 | 52 | 81 | Upper Kurast |
| Act 3 | 24 | 53 | 81 | Kurast Causeway |
| Act 3 | 24 | 54 | 82 | Travincal |
| Act 3 | 21 | 50 | 79 | Spider Cave (Arachnid Lair) |
| Act 3 | 21 | 50 | 79 | Spider Cavern |
| Act 3 | 21 | 51 | 80 | Swampy Pit Level 1 |
| Act 3 | 21 | 51 | 81 | Swampy Pit Level 2 |
| Act 3 | 22 | 51 | 81 | Flayer Dungeon Level 1 |
| Act 3 | 22 | 51 | 82 | Flayer Dungeon Level 2 |
| Act 3 | 21 | 51 | 82 | Swampy Pit Level 3 |
| Act 3 | 22 | 51 | 83 | Flayer Dungeon Level 3 |
| Act 3 | 23 | 52 | 84 | Sewers Level 1 |
| Act 3 | 24 | 53 | 85 | Sewers Level 2 |
| Act 3 | 23 | 53 | 84 | Ruined Temple (In Kurast Bazaar) |
| Act 3 | 23 | 53 | 84 | Disused Fane (In Kurast Bazaar) |
| Act 3 | 23 | 53 | 84 | Forgotten Reliquary (In Upper Kurast) |
| Act 3 | 24 | 54 | 85 | Forgotten Temple (In Upper Kurast) |
| Act 3 | 24 | 54 | 85 | Ruined Fane (In Kurast Causeway) |
| Act 3 | 24 | 54 | 85 | Disused Reliquary (In Kurast Causeway) |
| Act 3 | 25 | 55 | 83 | Durance of Hate Level 1 |
| Act 3 | 25 | 55 | 83 | Durance of Hate Level 2 |
| Act 3 | 25 | 55 | 83 | Durance of Hate Level 3 |
| Act 4 | 26 | 56 | 82 | Outer Steppes |
| Act 4 | 26 | 56 | 83 | Plains of Despair |
| Act 4 | 27 | 57 | 84 | City of the Damned |
| Act 4 | 27 | 57 | 85 | River of Flame |
| Act 4 | 28 | 58 | 85 | Chaos Sanctuary |
| Act 5 | 24 | 58 | 80 | Bloody Foothills |
| Act 5 | 25 | 59 | 81 | Frigid Highlands |
| Act 5 | 26 | 60 | 81 | Arreat Plateau |
| Act 5 | 29 | 61 | 82 | Crystalized Cavern Level 1 (Crystalline Passage) |
| Act 5 | 29 | 61 | 83 | Cellar of Pity (Frozen River) |
| Act 5 | 29 | 61 | 83 | Crystalized Cavern Level 2 (Glacial Trail) |
| Act 5 | 29 | 61 | 84 | Echo Chamber (Drifter Cavern) |
| Act 5 | 27 | 60 | 81 | Tundra Wastelands (Frozen Tundra) |
| Act 5 | 29 | 62 | 82 | Glacial Caves Level 1 (Ancients' Way) |
| Act 5 | 29 | 62 | 83 | Glacial Caves Level 2 (Icy Cellar) |
| Act 5 | 37 | 68 | 87 | Rocky Summit (Arreat Summit) |
| Act 5 | 32 | 63 | 83 | Nihlathak's Temple |
| Act 5 | 33 | 63 | 83 | Halls of Anguish |
| Act 5 | 34 | 64 | 84 | Halls of Death's Calling (Halls of Pain) |
| Act 5 | 36 | 64 | 84 | Halls of Vaught |
| Act 5 | 39 | 60 | 81 | Hell 1 (Abaddon) |
| Act 5 | 39 | 61 | 82 | Hell 2 (Pit of Acheron) |
| Act 5 | 39 | 62 | 83 | Hell 3 (Infernal Pit) |
| Act 5 | 39 | 65 | 85 | The Worldstone Keep Level 1 |
| Act 5 | 40 | 65 | 85 | The Worldstone Keep Level 2 |
| Act 5 | 42 | 66 | 85 | The Worldstone Keep Level 3 |
| Act 5 | 43 | 66 | 85 | Throne of Destruction |
| Act 5 | 43 | 66 | 85 | Worldstone Chamber |
Via [2]