A HubEngine Relation is used to merge the data records from two endpoints. Each data record that is retrieved via the endpoint of an API has an ID (identifier). In a plan, the IDs of the data records from the connected systems are pooled.
For example, the contact ID from the marketing automation system is related to the contact ID from the CRM system (MA-Contact-ID<>CRM-Contact-ID). If 1,000 contacts from two systems are kept in sync via a plan, there are 1,000 relations in the relation store.
Help Center: How can I manage the relations of a HubEngine Plan?