Short answer: “a type system centered on the use of set-theoretic types (unions, intersections, negations) that satisfy the commutativity and distributivity properties of the corresponding set-theoretic operations”.
Long answer, well, there are blog posts[0], the Design Principles of the Elixir Type System paper[1] and related presentations[2, 3, 4] that talk about it at length. Giuseppe Castagna’s site has many more related papers: https://www.irif.fr/~gc/topics.en.html
[0]: https://elixir-lang.org/blog/2022/10/05/my-future-with-elixi...
[1]: https://www.irif.fr/~gc/papers/elixir-type-design.pdf
[2]: https://www.youtube.com/watch?v=gJJH7a2J9O8
Sets and types are foundational mathematical concepts so I’m looking for how elixir’s types fit in that context. Union and intersection are not something that belongs only to sets.