The main entity types are relationships, interactions, interaction contacts and products along with product-category hierarchy information.

Relationships
Relationships are connections and associations between different entities such as customers, products, and campaigns. Contacts can have multiple relationships to one account, or relationships to several different accounts, each one using a different channel of communication. This data model helps to define and track the various types of relationships between these entities, such as a customer’s engagement with a specific product or their participation in a marketing campaign. This allows for a more comprehensive understanding of the interactions and connections between different entities, enabling better targeted and personalized marketing efforts.
You’ll learn more about this topic in the "Building Successful Contact to Account Relationships" lesson.
Interactions
Interactions are all transactional data that describe what happened by answering the questions: who did what, by which means, why and when. This is done using contact attributes, interaction type, communication medium, item of interest and a time stamp. The information about various consumer activities (such as issues raised by the user in Service) and sales-related activities (such as sales order data, web clicks, email responses, social media posts, and participation in events) is stored as interactions in the system. The relevant information is held in multiple database tables in SAP HANA database.
Interaction Contact
The interaction contact is the central master data object in SAP Marketing Cloud which can store contact and corporate account information. The data stored in the interaction contact is updated through a best record algorithm that collects data from a source system which contains all data fields (technically known as Facet Data).
The contact origin data contain all data fields and they are filled by the data coming from different source systems, such as ERP or CRM. The marketing attributes are available to store information as name-value pairs.
Product / Product Category
Now let‘s discuss the product and product category entities. You can categorize products, for example, pens, ink cartridges, and erasers which can be grouped together in the category Stationery. The Product entity contains all the products with their category assignments and description. The product category and products are master data entities which can be imported locally to be available in various marketing processes.
There are a few common-use cases which leverage the product and product category entities. You can use them to define recommendations, specify filters within Segmentation, and store information about sales orders and included line items. Further, the product entity can be used to replicate the order status from canceled orders as well as saved shopping carts.