A corpus (pl. corpora) is a collection of spoken or written texts to be used for linguistic analysis and based on a specific set of design criteria influenced by its purpose and scope. There are various, and sometimes conflicting, definitions in the relevant literature (...) as to what exactly constitutes a corpus, but for our purposes, we’ll adopt the relatively simple and straightforward one given above. This basically means that any collection of texts that has been systematically assembled in order to investigate one or more linguistic phenomena can be termed a corpus, even if it may only contain a handful of classroom transcripts, interviews, or plays.
What is the definition of linguistic phenomenon in the context of a linguistic corpus? I don't know what they mean by a phenomenon.