English someone can give rise to the speaker’s ignorance implicature (Somebody arrived late —> the speaker doesn’t know who). Some of its analogues in other languages, e.g. German `irgendein’ or Russian `kto-to’, have conventionalized this implicature into an inference. Namely, the ignorance inference became a part of their semantics. And it is these elements that Aloni and Port call epistemic indefinites (EIs).
The ignorance inference is the main focus of Aloni and Port’s papers. They are proposing an analysis that derives this inference with the use of Aloni (2001)’s theory of concepts and conceptual covers. They argue that EI represent a special case of domain widening.