Use impl Trait for static dispatch where the compiler knows the concrete type at compile time, and dyn Trait for dynamic dispatch where the type is resolved at runtime via a pointer. impl Trait is used in function arguments or return types to enforce a specific type without naming it, while dyn Trait creates a trait object that can hold any type implementing the trait.
// Static dispatch: compiler generates specific code for each type
fn process_static(item: impl Display) { }
// Dynamic dispatch: uses a pointer to a trait object
fn process_dynamic(item: &dyn Display) { }