Book and the ledger

Books sit at the bottom of a strict accounting hierarchy:

Legal Entity → Entity → Business Unit → Cost Centre → Ledger Book

Each ledger book belongs to exactly one Cost Centre; a Business Unit can have many Cost Centres; an Entity is made up of many Business Units; a Legal Entity is composed of many Entities. Books also carry desk/sub-desk, location, and senior-funder metadata used for ownership and dimensional reporting.

Mapping onto the ORE Studio domain model: no dedicated Legal Entity or Branch entity exists in the codebase, and none is needed — the existing party=/=business_unit pair already cover this hierarchy semantically. book.party_id (mandatory) is the Legal Entity: party is documented as "an internal legal entity participating in financial transactions". book.owner_unit_id (optional FK to business_units) covers Entity, Business Unit, and Branch alike — business_unit is deliberately generic ("desks, departments, branches") and self-referencing via parent_business_unit_id, so a branch is simply a business unit at the right depth in that chain rather than a distinct entity type. Cost Centre is separately tracked as free-form metadata on the book itself (see the ledger-book fields above), not as its own lookup entity. Investigated as part of the "Book data model cleanup" story; no new fields or entities were added as a result.

• Ledger books have their balances carried forward monthly as part of Month End close, with a special year-end variant (Year End) where opening balances for the new year are posted separately.
• Within the ledger, a book's trial balance is broken down by currency, further split by product (Repos, Loans, Spots & Forwards, Vanilla, Complex) and by counterparty type (Internal — e.g. IDT — vs External — other banks, customers). Per currency, the ledger tracks both functional and transactional Daily, Month-to-Date, and Balance figures.
• Forward valuations sent to the ledger must be expressed at the leg level.
• Each book has a functional currency, as designated by the ledger. Because trades happen in transactional currency, a daily automated process — the Ledger Sweep — spot-converts transactional-currency positions (live and settled cash) into functional currency, capturing the resulting P&L into a dedicated FX Revaluation entry (see FX Spot Revaluation Types for the three treatments that entry can take) for that book.
• Finance separately maps funding books to their associated ledger books and requires a book-level valuation report aggregating all MtM and settled cash into book-level results. Any P&L difference arising purely from the daily natural-to-functional currency conversion is posted as a distinct P&L Adjustment.

• Profit Sell Down — a monthly adjustment that crystallises profit out of a trading book. Two deals are booked — one in the trading book, one offsetting in a global/central book. Quarterly, the central book's balance determines how much cash is remitted to Group, after which an offsetting trade zeroes the central book back out. This central book is the Central Remittance Book.
• The Profit Remittance tool formalises this: a designated remittance book (e.g. Central Remittance) receives trades against a special Profit Remittance internal counterparty, entered per book. Because these trades only move cash balances, they affect funding but are booked as balance-sheet only and do not touch P&L.
• EOM Cleardown — at each month end, a cash trade equal to that book's P&L (in transactional currency) is booked, alongside a functional-currency (e.g. GBP) cash-equivalent trade for the book, with a matching gloss trade posted to the global functional book. Positive P&L books receive cash.
• Sweep — centralises long/short cash balances by sweeping them into a central book. Only books explicitly flagged as Sweepable participate.

Book-level attribution is a recurring backbone of reporting across the system: risk measures (e.g. Rho — the effect of a 0.01% rate move) are reported at book level; the standard P&L report is fundamentally "P&L by book" (Book, Book Currency, P&L Currency, FX rate, P&L, Delta, etc.); and a Control Explain process explicitly calls out book moves as a cause of hard-to-spot P&L breaks between days, since the deal's prior book is not retained on the deal record itself (see Book lifecycle).

Two ledger-adjacent control reports exist specifically for book static data: the Amended Books report (books whose static data changed between two time points) and the Book List report (as part of standard EOD reporting).