OptionalproductThe configured product: the parent, or any of its variants. One of productId/sku.
OptionalselectionsOptionalsetStart a new set with this group (from a product page: sourceType: "pdp"). Not with setId.
OptionalsetJoin an existing set ("Komplettera i samma tyg"): the set's choices for the profile are inherited (a different one answers 409 SET_SELECTIONS_MISMATCH), and the goods go into the set's group of the same product and choices when there is one.
Optionalsku
A product configured with its option profile, added as one group.
itemsare the goods (one per size or variant);selectionsthe choices per position. Charge lines are added by the server.