When Alice and Bob use a bilateral ledger protocol like SNAP or BTP to negotiate and record a transaction, the append-only message log acts as a ledger.
I always thought of Federated Bookkeeping as a federation of such ledgers. But a more universal view may be to separate transactions from their negotiation channels.
That way, the server-to-server communication in Federated Bookkeeping serves two functions:
1. Negotiating transactions between sender and receiver, in a way that avoids inconsistency.
2. Forwarding information about transactions that were added to the "World Ledger" in the past. This is then not restricted to transactions that were negotiated in that same channel.