Choose the correct domain boundary
BillingSpec models commercial parties, catalog/pricing, quotes, orders, invoices, credit notes, payments, reconciliation and related evidence. BillingRuntime supplies deterministic projections over those contracts with an injected tax-calculation port and backend-neutral persistence interfaces. The separate country package and provider integrations handle their specific responsibilities. Start by identifying the exact lifecycle and jurisdictional composition required by the host; a country-neutral operation declaration is not proof of configured production invoicing or payment execution.
Preserve economic history
The billing truth surface normalizes facts into an append-only evidence lifecycle with revision and idempotency checks. Corrections supersede earlier facts while retaining history. FinanceOS validates exact money, entity/tenant isolation, balanced postings, segregation of duties and budget freshness. Consequential work becomes a digest-bound handoff for host approval and execution. This division makes an accounting review reproducible: the reviewer can follow the original fact, its correction lineage and the proposed posting instead of trusting a mutable total.
Respect unit and execution constraints
Finance consolidation is holding-bound and homogeneous in currency and scale. Mixed units need governed conversion evidence and are denied by default in the documented runtime. BillingRuntime defines invoice, credit-note, event-log and invoice-chain ports rather than selecting a database or payment provider. Keep a proposed handoff, an authorized action and a confirmed external effect distinct in product copy and telemetry. Adapter qualification should address replay and ambiguous provider outcomes as well as the successful path.
Follow a practical path.
Trace one commercial transition
Start with a quote or invoice reference and follow the declared operation, lifecycle projection and evidence event. Identify the exact money representation, tenant/company scope and persistence port involved.
Inspect a correction and handoff
Follow a superseding correction into the resulting finance proposal. Check balanced units, required evidence and the Connect authorization reference before considering the handoff eligible for a host executor.
Choose the external boundary
Locate the country and provider adapter used by the intended product. Review its credential, submission, reconciliation and failure responsibilities separately from the portable runtime; qualify that composition in its owning environment.