The aggregation of e.g. a project, site, building, building storey… can be considered one of the possible hierarchies. The IFC schema is represented as a graph of linked entities, but most viewers display the Spatial Hierarchy as a tree, because it is a very good representation of the spatial relations.
As already answered, IfcZone is a parallel grouping, which is not strictly part of the Spatial Hierarchy. Some viewers show grouping as an alternative hierarchy, which would show also Systems (another form of grouping).
Within your second question, yes, there may be items below IfcWall, when there are e.g. Building Element Parts.