This is easy
Every entity in the system, representing the human-world object which the system is expected to interact with is a business object.
The human-world logic in the system around manipulating business objects is a business logic.
This is opposed to the objects and logic being part of the implementation details.
For example in the multi-tier trading system order/trade will be business objects, but heartbeat will be implementation detail.
The logic of encoding order/trade objects for streaming will be implementation detail, while the logic of changing the order state to ‘completed’ once trade arrives is a business logic.