> `box` only means heap allocating in Rust and we're are talking about learning the language.

Not so, "boxing" is the term of art for indirect heap-allocated structures in FP languages implementations and — although the meaning is somewhat broader[0] — in OO languages with a visible distinction between "object" and "primitive" types such as Java and C#.

[0] it includes both heap-allocation and wrapping in a full-fledged object