The type of regulatory_book is defined in Article 104 of the CRR
The trading_book and banking_book enum values have extended definitions provided by the BCBS 457.
The BCBS 457 paper goes in to more detail but here is an exerpt:
25.2 Instruments comprise financial instruments, foreign exchange (FX), and commodities. A financial instrument is any contract that gives rise to both a financial asset of one entity and a financial liability or equity instrument of another entity. Financial instruments include primary financial instruments (or cash instruments) and derivative financial instruments. A financial asset is any asset that is cash, the right to receive cash or another financial asset or a commodity, or an equity instrument. A financial liability is the contractual obligation to deliver cash or another financial asset or a commodity. Commodities also include non-tangible (ie non-physical) goods such as electric power.
25.3 Banks may only include a financial instrument, instruments on FX or commodity in the trading book when there is no legal impediment against selling or fully hedging it. 25.4 Banks must fair value daily any trading book instrument and recognise any valuation change in the profit and loss (P&L) account.
Essentially defined as everything that is not in the trading_book.