The term "formulaic sequence" (FS) is used with a multiplicity of meanings in the SLA literature, some overlapping but others not, and researchers are not always clear in defining precisely what they are investigating, or in limiting the implicational domain of their findings to the type of formulaicity they focus on. The first part of the article provides a conceptual framework focusing on the contrast between linguistic or learner-external definitions, that is, what is formulaic in the language the learner is exposed to, such as idiomatic expressions or collocations, and psycholinguistic or learner-internal definitions, that is, what is formulaic within an individual learner because it presents a processing advantage. The second part focuses on the methodological consequences of adopting a learner-internal approach to the investigation of FSs, and examines the challenges presented by the identification of psycholinguistic formulaicity in advanced L2 learners, proposing a tool kit based on a hierarchical identification method.