From OpenCog

Next, we use the term 'Unit Group' to refer to a multipart AtomSpace, each component of which is associated with some CIM-Dynamics, and which as a whole may also be associated with a collection of CIM-Dynamics. In theory a Unit group could be a grouping of groupings of groupings of ... groupings of AtomSpaces. In the current OCP design it never gets all that deep. For instance, we have the Evolutionary Programming Unit-Group, which contains a set of Unit-Groups corresponding to evolving populations with different fitness functions. And in a full-on self-modification OCP configuration, we will have Unit-Groups corresponding to whole OCP systems, each being studied and optimized by a ìglobal controllerî OCP contained in its own Unit-Group. This results in a maximum depth of three: So, we have, in the worst case that seems feasibly likely to be necessary for human-level AGI, a Unit-Group that is a set of sets of sets of simple AtomSpaces.