I shied away from the word "kind" simply because that has a specific meaning in type theory that doesn't fit here. Higher-kinded types are a must-requested feature for Rust, and I didn't want to confuse readers into thinking this has some connection to Rust's string types.

The word "sort" isn't free of alternate associations either, and "kind" seems less likely to confuse people, seeing as Rust doesn't have any concept called "kinds" right now and won't have HKT for years, if ever.

"sort" also has a specific meaning in type theory - it's a class of kinds.

Also the type-theoretic meaning of "kind" is accurate here, no? Owned vs slice really are two different kinds - it's just that Rust can't represent kindedness in general (it has specialized support for tracking ownedness, but you can't reuse that system to track other properties).