—A Villager in Prologue III chapter in Fire Emblem: Shadow Dragon
Usage refers to the preset number of times a weapon, staff, or item can be used. Fans of the franchise often refer to the mechanic as Weapon Durability as upon using all usage points of a weapon/staff, they are given a message that "[Weapon/Staff] Broke!"
This heavily deviates from most RPGs as this gives weapons and staffs greater importance in battle as players are discouraged from simply running through levels utilizing the strongest weapons/staffs possible, instead maximizing each weapons usage on characters to ensure that they are able to effectively battle common enemies and use stronger weapons for crucial fights.
Most weapons acquired have around 20-45 uses before they wear out and disappear from a character's inventory. A strategical aspect of playing the games is knowing how to kill enemies with as few attacks as possible in order to save the uses of weapons; money may be very limited, so weapons cannot be replaced on a whim. The durability of most weapons is graded on its strength and availability: the common Iron weapons and Steel weapons have the highest number of uses, whereas the superior Killer weapons and Silver weapons have a scant 20 uses, and rare items, such as the Silence Staff and the Bolting Tome have only 3-5 uses.
There are, however, some exceptions to this rule: the starting weapon unique to the Lord character of a specific game is usually a versatile and powerful weapon that has a decent number of uses (some examples include the Mani Katti, the Rapier, and the Reginleif). Other times key weapons are exempt from Usage and simply may be used an infinite amount of times such as Falchion, Ragnell, and Amiti, usually due to story importance and/or some form of divine blessing that prevents weapon wear.
The rare Hammerne Staff is able to restore full usage to most weapons, but due to its own limited durability, the weapons it is used on should be chosen wisely, as once it is completely used up, the Hammerne staff will break and cannot be repaired. The Armsthrift skill introduced in Fire Emblem Awakening can also help mitigate weapon usage. Missing attacks also does not reduce the durability of weapons, since of course the enemy was not struck. An exception is magic in some games, which does use durability when it misses, as the spell was still cast.
Counter attacking with a staff doesn't consume a usage of the staff.
In Fire Emblem: Genealogy of the Holy War, players can get their weapons repaired at castles they own. When a weapon has been fully used, it will also become a broken version of that weapon that the player can still use to attack, but has a lot less power and accuracy. All weapons but the Earth Sword and some staves have 50 uses in this game, with noted weapons having only 10.
In Fire Emblem Gaiden, Fire Emblem Fates, and the remake of Gaiden, Fire Emblem Echoes: Shadows of Valentia, there is no durability mechanic, allowing players to continuously use the same weapon without having them break. However, Staves in Fates still have a limited number of uses. Fates offsets the lack of durability with side effects from using certain weapons, usually containing heavy drawback for using stronger weapons. In Shadows of Valentia, characters learn Weapon Arts as they grow proficient with a specific weapon, but will lose access to Arts that they have learned should they switch to a different weapon.
In Fire Emblem: Three Houses, the weapon durability system is similar to that of Genealogy of the Holy War. Weapons have a limited durability, but upon breaking are still usable with reduced stats, namely lower hit and mt, and enemies are likelier to perform follow-up attacks against characters. Broken weapons can be repaired for an expense of gold. Characters can also learn and use Combat Arts, but their usage costs high amounts of durability.
In TearRing Saga: Berwick Saga, the way durability works is changed. Throwing weapons have a limited number of uses, like in the rest of the series, while magic has MP that slowly regenerates after each chapter. Non-throwing physical weapons have a set 100 durablity that decreases with each use depending on durablity rating (F-7, E-6, D-5, C-4, B-3, A-2, S-1). Each weapon also has a chance to break after every attack depending on its durability (100-61: blue - 0%, 60-41: green - 1%, 40-21: yellow - 2%, 20-1: orange - 4%, 0: Red - 100%). Bows, Crossbows, and Ballistae use both Durablity (for weapon itself) and number of uses (for ammunition). Like in Genealogy of the Holy War, it's possible to repair weapons for a fee, so long as they don't completely break.