Hierarchy can be [1 p. 83]:
- order hierarchies - ranking on some criteria;
- inclusion hierarchies - "Chinese boxes" aggregations;
- control hierarchies - who gives orders to whom; and
- level hierarchies - ontological organisation, in which entities exist at different levels.
perhaps also classification hierarchies.
Complex entities are composed of inclusion hierarchies that are nearly-decomposable [1 p.86]. These components are loosly coupled bith vertically and horizontally. Processes at different levels have different time scales.
A key concept in nearly decomposable systems is that of the ''small world'' [2]. This means most nodes are only connected to a small community (where everyone knows each other) but a small number of nodes are connections between these communities.
Where the connections per node follow a Pareto [power] law, these are called 'scale free' networks [1 p. 227].
Latour (see Actor-network Theory) rejects level hierarchy [1 p. 107]. However, ANT uses Punctualisation, which is an inclusion hierarchical concept, so care must be taken with considerations of hierarchy.