Except for
## IsLibrary: true , there is no underlying difference between normal add-ons and libraries. They both can do the same things, both can have UI (up to and including windows and settings pages). They both can register event listeners. They both can affect your FPS while in Cyrodiil.
An add-on such as
Votan's Addon List can help identify which libraries are not currently used by any active add-ons.
An add-on such as
Circonians Addon Selector can help create sets of add-ons, such as one set for crafting and one for PvP.