Ledger, Kernel, and Retainer OS have separate financial responsibilities. Ledger owns accounting-provider integration and evidence; Kernel owns accepted provider-neutral invoice and payment facts needed by multiple consumers; Retainer OS owns engagement-scoped retainer policy and calculations.
Why the layers stay separate
An invoice reported by an accounting provider is evidence. Once reconciled and accepted under the applicable workflow, a provider-neutral invoice or payment fact can become shared canonical truth. Retainer OS then combines accepted facts with versioned policies such as periods, rollover, overburn, discounts, delivery cost, and margin formulas.
A Billing Summary is a composed read model, not a replacement for the accounting record. Retainer OS should not be described simply as a billing system: its distinctive responsibility is explainable retainer economics and profitability projection.
Implementation depth varies. The adopted boundary is not a claim that every provider, reconciliation path, multi-currency rule, or forecast is complete.