Subordination is one field: an issuing CA is created with issuerRef naming its parent, instead of the empty value that means self-signed root. But the field worth slowing down for is the optional one next to it.
A CA is more than its subject and algorithm: labels, path-length constraint, CRL cadence, rollover policy — and the publication URLs that end up embedded in every certificate (CDP/AIA). Historically those could only be set after creation, and the gap is not cosmetic: a CA that issues for ten minutes without publication URLs embeds them in nothing, and those certificates carry the omission for their whole life.
So the create form has a second, optional half — + Additional attributes — a spec block in the same document: labels, constraints, crl, publication, rollover. Over the API it is one field, spec, in the POST /v1/cas body.
It only adds: fields the form itself owns (subject, algorithm, custody, parent, validity) are refused inside spec, by name — two writers for one field means one wins silently, and the refusal arrives before any key is generated.
Now I can create a subordinate CA with its publication URLs set from minute one.