Excuse me Sir/Madam, which Model?

Abstract Development of complex systems through the application of Model‐Based Systems Engineering (MBSE) will require the creation and maintenance of multiple models, created using multiple languages. Hence, structuring the models such that there is efficient support for incremental development requires some foresight. Having models with multiple or unclear purposes may introduce situations where parts of the organisation will ask for modification for representing a desired future system state, whereas other parts of the organisation require it to remain unchanged in order to represent the present state. This paper introduces a structure with associated terms and characteristics of the different model types in order to provide guidance to an organisation. In particular, it extends previous work on model tenses, capturing different time perspectives/purposes on the models, to also include model usage for keeping track of the actual state of a realised system instance. For example, the concepts of Digital Twin and Digital Shadow are discussed and conceptualised. As such, the contributions made in the paper allows an organisation to reason about the completeness of their modelling methodology and its applicability to support incremental development. Above all, the paper spells out clearly that for a complex heterogeneous system, any request for an update to “the model” will prompt the response “Excuse me Sir/Madam, which model?”.