@stuartyeates Endeavor is precisely defined in https://www.dublincore.org/specifications/openwemi/specification/ as a superclass of W, E, M and I. It's intended to be used where you want to refer to any of W, E, M and I. You can think of it as a union type, if you like.
Super-work entities like the DC Universe are outside the scope of the #OpenWEMI project, but nothing prevents an implementer using OpenWEMI adding a super-work entity.