I think it would be useful to document some of the details of currently implemented VOS meta-objects that don't appear in the API documentation. This especially includes "external" aspects of the metaobjects such as their messages, child objects/properties, access control policies, and so on, that can be seen by the outside world connecting to a VOS server, regardless of their implementation in the internals of the VOS library code.
Eventually (in VOS S5) this documentation will be autogenerated from the ObjectTypeDefinitions.
So here's a start (this is not a complete list... please add more!):
A3dlActor
A3dlBillboard
A3dlClone
A3dlCone
A3dlCube
A3dlCylinder
A3dlLight
A3dlMaterial
A3dlModel
A3dlObject3d
A3dlPolygonMesh
A3dlPortal
A3dlSector
A3dlSnow
A3dlSphere
A3dlStatic
A3dlTexture
A3dlViewpoint
MiscAvatar
MiscCompoundFactory
MiscFactory
MiscHypercard
MiscIrcUser
MiscMetadata
MiscSearch
MiscStoredObjectsFactory
MiscTalkative
MiscTextEntry
PropertyProperty
These aren't metaobjects, but they have messages and access control policies that can be documented the same way: