Requirements

Requirements are prerequisites that a character must fulfill to wear an item. If these requirements are not met, the item cannot be equipped and used. Generally, the more powerful an item is, the higher are its requirements; in addition, some items' requirements are adjusted to reflect general progression of a specific class (such as caster items requiring high Magic, but low Strength).

Diablo I-II
Strength requirements are the most common, but Dexterity is also seen on items like bows and other ranged weapons, and big weapons like polearms and two-handed swords.

Subtracting or adding requirements is an attribute which can be found on any piece of item, including gems.

This attribute can either deduct or increase the amount of requirements needed to equip an item by a percentage. The Required Level to use the item, however, can't be reduced.

Example: a chest piece that requires 50 Strength and has one socket. A gem that reads "Requirements -10%", if placed into the chest piece, will instantly make it require only 45 Strength.

Diablo III
In Diablo III, the only requirements on items are character class and character level. The latter can be reduced by having the appropriate secondary affix on an item (up to -30 on level 70 (-40 with Ancient Legendary bonus), allowing to equip a level 70 item on a level 40 (30) character).

As of patch 2.0., all legendary items have a minimum required level to drop and equip, but their required level changes depending on that of the player's own level at the time of drop. On very rare occasions, one can find an item which is slightly higher on level. The only equippable level 60(70) items that do not have required level are Hellfire Rings and Hellfire Amulets.

One may bring items to required level of 1 with the help of Gem of Ease: either directly or by transmuting it in the Kanai's Cube.

In addition to class-specific items, all classes have a fixed set of regular weapon types they cannot use: for example, Wizards cannot use Polearms. Non-class-specific armor can be equipped by anyone.

Follower items can only be used by Followers, and may never be equipped by player characters.