What is the difference between the ExternalFn and Fn expressions?

[CrowdHailer]

I've been working under the (i assume) incorrect assumption that the difference here relates to whether the functions are public or not. I've just discovered a public field so can't work out what ExternalFn vs Fn means.

https://github.com/gleam-lang/gleam/blob/main/src/erl.rs#L189-L201

[lpil]

Fn is fn main() { do_stuff() } while ExternalFn is external fn main() -> Int = "module" "function".

Some doc comments could help here for sure