The OCaml manual states:
As another optimization, unboxable record types are represented specially; unboxable record types are the immutable record types that have only one field.
(https://caml.inria.fr/pub/docs/manual-ocaml/intfc.html#ss:c-tuples-and-records)
But which type can that "one field" have? Only native, or any record type?
Any type: OCaml memory representation is uniform.
More precisely, in term of memory representation, OCaml values are either integer or pointers to a block. And a block consists in a header followed by a number of values.
Unboxing replaces the unboxed memory representation of a pointer to a block which contains only one OCaml value by this value.