OptionaldefinitionCaller-supplied initial-field entries — initialFields for a direct
start, the parent's projected with bindings for a spawned child.
The instance document _id — the per-instance join key (engine-minted
<tag>.wf-instance.<random>; caller-supplied ids ride verbatim). One of
the payload policy's two deliberate customer-string exceptions — see the
module doc.
Started by a parent's subworkflow spawn rather than a direct
startInstance — automation vs. hands-on at instance creation.
Absent only when the definition predates content fingerprinting.