a new car is manufactured, or a new job is created. It can also begin when an existing unit is vacated by someone leaving the system under consideration, such as an employee retiring, or a home owner goes to a nursing home.

Eventually a chain will come to an end, usually with a new entrant into the system or when the last unit in a chain is abandoned, merged, or destroyed. This can involve a new employee recruit or a housing unit on the market for the first time, or when a house in the chain is torn