This API is designed for third party mod developers to add compatibility with both the Dynamic Sword Skills and Zelda Sword Skills mods.
As such, it will be required to be installed in order to run the 1.9.4 and higher versions of both DSS or ZSS.
For mod developers, use this API if you have an Item that you wish to control how it interacts with sword skill use or a custom DamageSource that you wish to control how it interacts with sword combos, for example.
To do so, simply include the deobfuscated .jar in your /eclipse/libs or a similar folder and add it to your build path, then implement any necessary interface(s).
Why are API always separate from the mods that require them...
In reply to caemyth95716:
Because it allows other devs to use the api's to create their own mods- it's an act of kindness to the modding community.