Fusion Material EXP is a weapon seal in Hyrule Warriors: Age of Calamity. It increases the experience gained when the weapon is fused to another weapon. The higher the weapon with the Fusion Material EXP seal is leveled before it is used as fusion material, the more total experience will be added to the weapon being enhanced.
Fusion Material EXP is not transferred to the weapon being enhanced when the weapon with the seal is used as fusion material, even when the seal is in a position where it would normally be transferred. This allows for leveling a weapon quickly to 20, 25, 30, or 50 without affixing any seals to it.
Note: A weapon receives a +5 damage buff from having any two weapon seals of the same suit, a +15 damage buff from having any four seals of the same suit, and a total +20 damage buff for having stacks of four seals in one suit and two seals in a different suit. Six seals of the same suit will only give a +15 damage buff. Therefore, if you wish to plan your weapon build in advance, it is important to know the hidden seals of your chosen weapon.