System-centred approaches consider language as an object in itself, which are expected to be studied from different angles corresponding to the various strata of the language system, such as sounds, graphical signs, groups of sounds, groups of graphical signs, lexical unities, syntactic entities, sentences and discourse. They can involve both synchronic and diachronic dynamics, concern the disciplinary contributions of philology, grammar, linguistics (considered as language science) and the specialisation thereof, for example, phonetics, phonology, lexicology, semantics and morphology, and are related to sciences such as semiotics, anthropology and sociology.
Ability-centred approaches focus on the mechanisms of language processing on an individual scale, which stem from diverse functional angles (i.e. aspects of cognitive control, perception, production, etc.) and can take into account different types of phenomena (sound and graphic production, biophysical signals, macroscopic behaviour, etc.).
Speech acts involve all identifiable modalities (production, reception, interaction and mediation), both in writing and orally. They can occur within various communication contexts, for example, in laboratories, families, care institutions, the workplace, prisons and schools, as well as under various types of pressure, including cognitive load, time pressure and adverse conditions of communication, stress and mood fluctuations, all of which can have an influence on speech. They can be delivered by speakers placed in situations of disability (acquired, functional and/or developing congenital diseases) and also by mono- or multilingual speakers using their first, second or third language within or outside a multilingual environment, whether or not they have been the object of specific procedures of linguistic planning. The prospect can also be static (where language is considered stable) or dynamic (referring to language acquisition, development, rehabilitation and attrition).