A relationship is a bi-directional representation of a real-world connection between two entities, for example a portfolio and a portfolio manager.
A custom entity extends the data model to represent a real-world object or concept that LUSID does not natively represent, for example office location.
A person entity represents a person of interest in your LUSID domain, for example a portfolio manager, trader or sales representative.
A legal entity represents an institution of interest in your LUSID domain, for example a bank, investment firm, pension provider and so on.