properties /identity

How a shoe's heel/back is constructed — whether the heel is enclosed, open with a strap, or absent — bucketed into the constructions footwear is sold and filtered by: closed-back (a full heel counter encloses the heel; the default — pumps, sneakers, boots), slingback (the heel is open, held by a strap across the back of the heel/Achilles; found on closed-TOE pumps and flats, not only sandals — a Manolo BB or Chanel slingback pump is a closed-toe shoe), and mule / backless (no back at all, the heel is fully exposed — mules, backless loafers, backless slides; 'open-back' is the same construction). It is statable of ALL footwear (Footwear, Boot, Sandal) and sits on the Footwear root. Minted under the code `back_construction` (not `back_style`) because `back_style` is already an APPAREL property — the shoulder/back cut of a sleeveless top (racerback/halter/scoop-back) — a homonym on a different branch with a different vocabulary that must not merge with this. Distinct from `closure` (the fastening mechanism — a slingback pump can be closure=buckle AND back=slingback at once), from `strap_type` (the sandal footbed strap configuration, where slingback was previously and wrongly filed), from `heel_height` (the heel's height tier) and from `toe_style` (the toe end of the shoe). For a backless SANDAL (a slide), the back coverage is already conveyed by `strap_type`=slide, so `back_construction` is left unset there rather than double-encoded as mule; it is stated where it is the distinguishing construction (slingback pumps/flats, closed-toe mules). It is an IDENTITY discriminator, not a buyable facet: on real catalogues back construction is a CATEGORY filter that navigates to DIFFERENT listings (a mule and a closed pump, a slingback and a closed-back, are distinct products each with its own SKU/reviews), never a single-product buy-box toggle — a given shoe simply IS a fixed back construction. Grouped in the `identity` concern, so by the derived-axis rule it is NOT axis-eligible — a seller never offers one shoe across back constructions as buy-box versions. Optional and single: a shoe has exactly one back construction, but most closed/athletic listings omit it (absent reads as closed-back) and only slingback/mule listings state it; reused across all footwear.