specialized: always substitute every type variable until they are all gone, creates many different "specializations" or "instantiations". This is like Rust

erased: just ignore the type variables for compilation. This is like the typescript -> javascript compilation. (Also how Java, Scala, Haskell, OCaml, etc. type paramters work)

Existential: "there exists a type such that..." see https://en.wikipedia.org/wiki/Existential_quantification https://en.wikipedia.org/wiki/System_F