Consumables are objects that add/restore stats and buffs to characters.
A typical consumable(a healing potion) is a class of items that can be hotkeyed and used without opening inventory.
Key insights:

1.Consumables are 'temporary boost' items, they don't bring permanent buffs or effects and disappear after use.
Consumable types split into
1.1. Stat restoring potions: healing/mana/etc which add points to depleted stats.

1.2. Buff consumables: grant temporary protection/boost/effect that expires after N seconds.

1.3.Charges of X skill/spell bound on items, throwable/fireable(e.g. arrows/bullets) item stacks are classified as weapon components and are not "true consumables".
This is often an anti-pattern gold sink that forces replenishing weapon/skill charges/components at shops and brings annoyance when they run out in middle of battle.
Proper alternatives:
1.3.1.Charges of X skill/spell would grant +x to Skill(instead of charge)
1.3.2 Charges/Arrows/Throwables self-recharge fast and cannot be depleted fully(bonus:grant more stats at full charge)

1.4.Quest consumables may bring permanent +stat/+skill boosts, but a better design would grant these effects on completion("You gained +4 skill points").

2.Consumable-centric games(Diablo series and similar) have a tendency to devolve into potion fights or attrition battle, which ends as consumables are spent. Having reserved space in inventory for potions is a common symptom.

3.Consumables stack their effect or duration when consumed in sequence(e.g. several healing potions at once).
This should be balanced with inventory size(which acts as power multiplier due #2).

4.Games which don't have consumables usually compensate with 4.1.high regen passive skills(sets mana/life regen higher per each skill point)
4.2. stat conversion skills(that consume mana/energy to grant buffs:e.g. selected aura that consumes mana to grant effect X)
4.3. plain passive buffs(effect X always on, stats of X are dependent on skill level).
4.4. items which grant the effect of consumables continuously or limited to N seconds(with recharge/repair )(see section 1.3).