The central configuration object for the settlement system. Defines a reconciliation relationship between two specific sub-accounts for a business line — e.g. mainapp:spot ↔ binance:spot. References Accounts (not Instances) so the exact sub-account type is unambiguous — one Instance (e.g. Binance) can have multiple Accounts (spot, margin-cross) and therefore multiple Settlement Lines. Every trade event, journal entry, position calculation, and risk check happens in the context of a Settlement Line.