Bold Hypothesis: GO BPs can be cleanly separated into cellular processes and larger processes formed from assemblages of these. Any process should be diagrammable. If the diagram is expressed using cell parts and molecules, it is cellular. If the diagram is expressed using cells or tissues it is at a larger level of organization.
Additionally, we may have grouping classes which can abstract over both branches, in terms of some objective or phenotype. Examples are: growth (to increase the size of the organism or a part), immune system