Business Layer covers functioning of the business, abstracted from software-intensive implementation. That layer is defined to ensure widest-possible range of software implementations. The word “system” as well as names of individual systems are banned from that Layer.
Furthermore, Business Layer models must abstain from assuming computers – after all, say contemporary retail banking was invented by Giovanni Medici in 1397, and was functioning for nearly six centuries before computerised.
The other way of looking at Business Layer is Customer Focus. After all, the Customers do not care what systems the enterprise implement – they do however care about how much legwork they need to do, what information should they provide or operate with, etc.
At the same time, the Business Layer models should be quite specific. Just like say postal address does not tell you which way to take yet ensures arrival of a letter to your home, the Business Layer models should specify precisely what will Customers, Regulators etc get., without explaining how the enterprise goes about delivering such capabilities.