Modelling levels for actor activity:
2. Operational level

The concepts chosen at the conceptual level acquire a more efficient expression though a context-based formalism at the operational level. We also retained from Cognitive Sciences the notions of mental representation and mental models, but with a different interpretation on the relationships between them. An activity is more than a task model, because it integrates how the task is realized and actor’s reasoning held. The context-based formalism provides an expression of actor's experience as a mental representation that brings together all activity developments made in different contexts. For simplifying the introduction of context from different sources in activity modelling, we assimilate context to a set of contextual elements. A mental model is an internal representation of external reality (Craik, 1943) to anticipate events, and a mental representation results of the accumulation of mental models obtained in different contexts. An actor, facing a known activity, does not seek to have a global picture of the activity but wants to follow step-by-step the reasoning to detect if all elementary decisions were justified in the context at hand or how a reasoning step must be changed.
The actor develops a mental model based on identification of the relevant contextual elements and the recovery of their instantiations in the context at hand. Identification and instantiation of contextual elements are part of reasoning from one step to the next one. This step-by-step evolution of the activity produces an ordered sequence of instantiated contextual elements that we call hereafter the proceduralised context. Proceduralised-context building, first part of decision-making, concerns the gathering, assembling and structuring of instantiated contextual elements. The proceduralised context expresses a real-time context, which evolves jointly with the mental model development.