External Entity Types are user-defined objects that allow you to associate and track any type of entity with your journeys. As opposed to Entity Types, which represent Chassi's native entities, External Entity Types are customizable and can be tailored to any type of object you want to track. There are two External Entity Types values already pre-seeded for your convenience: customerId and contactId.
For a detailed guide on how to utilize external entity types, see Using Entities with Journeys .