This is a model of the 'social formation' (Althusser's term for an actual society). It is hinted at in the famed ISA's Essay (reading guide here), where Althusser rethinks the base/superstructure model in terms of a house -- the house has foundations but each floor or 'level' can be laid out quite differently (some will have two rooms, others three, some will be cuboid, others will have pitched ceilings and so on) -- they will be 'relatively autonomous'. Of course, the foundations will still limit to some extent what can be built on the floors above -- it will determine the architecture 'in the last instance'. However, we now have a more sophisticated model, as Hall goes on to explain.
With this homely analogy, we are
set to develop a more complex model of a society. There are three main
'levels', indicated by the initials -- E for economic level, P for political
level, and I/C for an ideological/cultural level. Of course, these floors
are still built on a foundation -- the mode of production, also called,
somewhat confusingly, the 'economic level' again. A distinguished colleague
of mine used to draw this 'levels' or EPI/C model thus: