Inline related resources on a single PayeeEngagement read (the payer-side flat path and the payee-nested path). Requirements inlines the Requirement instances attached to this engagement, each stamped with blockingMode, and is available to the payer seat. WorkDefinitions inlines the WorkDefinitions attached to the underlying Engagement template and is available to both seats — the template is payer-owned, so this is the payee's only read path to the attribute shape a WorkItem must satisfy. Unsupported values return 422 instead of being silently ignored. Use this expansion for the per-engagement view; the retired /engagements/{id}/requirements collection is not a V3 route. For filtered or paginated reads, use /v3/onboarding/payee-requirements?filter[payeeEngagementId][in][]=...; those standalone rows intentionally omit the engagement-contextual blockingMode. Expansions are not available on list/search rows.