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Users create Subscriptions to container visits using a combination of Container, Category and Port. When PortConnect finds a container visit that matches the specifies combination a subscription then PortConnect starts sending the events that have been registered for.
E.g. a user subsribes subscribes to receive the Active event against Container XXX which is an Import at with a category of Import and a port of Port of Auckland. If/when an Import container called XXX is created in the Port of Auckland system then PortConnect will send the Active event.
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