Modelling levels for actor activity:
4. Environment level

The immediate environment of an activity is all that is not in the activity but constrains its development. The two elements at this level are the sources of context and the instantiation of a contextual element. The four sources come from the actor, the activity, the situation and the available resources needed in the local environment. They provide in routine the instantiations for contextual elements on the path followed in the contextual graph. Actors play a central role in activity modelling because knowledge in a contextual graph is mental representation and experience of the actor. Moreover, the actor is responsible for fixing unexpected situations. First, the mental model is correct, but its context is new for the actor and thus not in the contextual graph. Second, a contextual element is missing in the model because it kept the same instantiation in all previous contexts (the constant instantiation is integrated in the activity before the development). Third, the activity must be performed in a radically different context that requires an extension of the activity model.
For efficient decision-making, actors seek first to identify the context at hand to determine the sequence of actions to realise and act rapidly. The context of a mental model can be analysed by unfolding the proceduralized context, and the “what to do” is provided by the list of actions to execute.
Garcia and Brézillon (2018) proposed a tree representation of an activity model, based on the series-parallel structure of contextual graphs, to interpret a mental model more easily than the graph representation. In this tree representation, the tree corresponds to the mental representation, and each branch corresponds to a mental model. Thus, mental models present two parts: diagnosis and action. Diagnosis part is the proceduralised context, and action part contains the “macro action” corresponding to this specific context. In several domains, operators reason first on the context of the problem to fix, and, second, prepare their decision -making according to the context.