See builtin generators for examples
Custom EQS item types
Custom EQS generators can return new kinds of EQS results. This can be done by extending the builtin UEnvQueryItemType classes. For example, see UEnvQueryItemType_SmartObject.
Custom item types which have locations should extend the existing vector-based one. This allows most of the builtin tests to work as they work on locations. You can create custom items without extending the vector-based one as well, but this almost certainly requires creating Custom EQS tests as well.