Book Currencies

Where every other currency role documented under Currency answers a question about how a given position is expressed, book currencies answer a prior, gating question: is this instrument's currency even eligible to be booked here at all? An instrument whose currency falls outside a book's permitted set cannot be booked into it regardless of how its denominated currency or functional currency would otherwise be resolved – the constraint is checked before any of those roles come into play.

Distinct from its use elsewhere as an older synonym for aggregation currency, a book's own Accounting Currency is a concrete per-book data field: the currency in which P&L adjustments specific to that book are stored. The book data model carries this attribute alongside its functional, reserve, ledger, and funding classification flags, which is worth naming explicitly precisely because the term "accounting currency" is overloaded – the same two words mean a book-level storage field in this context and a risk-engine-wide convert-and-compare currency in the aggregation-currency context, and the two must not be conflated when reading either concept back into the other.