A B/L does three jobs at once: it is a receipt for cargo on board, a contract of carriage and, above all, a negotiable document of title. Whoever holds the original can claim the goods at destination.
There are variants: "to order" (negotiable and endorsable), "straight" (consigned to a named party) and "telex release" (digital release with no physical originals). Each carries different legal and practical consequences.
Day to day, tracking B/Ls — received, endorsed, handed to the bank under a letter of credit — is where deals most often stall, and where a CTRM earns its keep by making the status visible.