Mods have minimum level requirements. The frame or weapon must have at least the given level before it can be equipped. Mods are generally broken up into 5 categories: Warframe, Rifle (also works for Sniper Rifle), Shotgun, Pistol, and Melee. Many mods are frame or weapon specific. That is, they will name an individual weapon or frame they can be attached to. The " 'Excalibur' Mod" can only be added to the Excalibur frame and no other frame. The " 'Skana' Mod" can only be added to the Skana and no other melee weapon.