Belt slots are spaces where potions and scrolls may be kept in Diablo and Diablo II instead of the main inventory. Some or all belt slots have number hotkeys available (1-8 or 1-4) for quick use of the items in the slots. Diablo II added the ability to expand the initial four slots by equipping better belts, providing more space and a mechanism to replenish slots automatically.
Belt slots represent the concept of a "utility belt" with multiple items strapped to it for quick access.
Belt slots did not return for subsequent games, which significantly reduced the emphasis on consumable items.
Diablo
In the first game, 8 belt slots are available, used with keys 1-8. Another hotkey uses health or mana potions chosen automatically, first from the inventory and then from the belt. Belts themselves are not an obtainable item type in this game.
Diablo II
The second game reduced the number of hotkeyed slots to four, used with keys 1-4. However, in addition to the "bottom" four belt slots showing in the interface most of the time, additional rows or levels of belt slots can be obtained with belts equipped, with higher quality belts providing more additional rows. Replacement items can then be queued up in additional slots "above" the main four, which "fall" one by one into the hotkeyed slots when the previous item in that slot is consumed.
A player may have one to four rows of belt slots with each row providing space for four items, and lengthening the queue of replacements for each slot by one. For example, with three rows of belt slots total, the player has 12 slots total, and may have a health potion prepared in slot 1 with two more prepared "above" it. Pressing 1 three times would consume all the potions, at which point that slot would be empty.
The number of rows available depends on the currently equipped belt, with the number of belt slot rows generally increasing with item level. Plated Belts are the first available belt that provides 4 rows of belt slots.