What is Main Data Object?
The main data object is a Salesforce Object from which the template will collect data and define the relationship with other Objects. If you want to create a document in which most of the information coming from a specific record or related to that record, then the object holding that record is the Main Object.
You want to create offer documents. The document contains information about an opportunity record and it's related information such as company name (Account), detailed offers (Opportunity Line Item). Thus, the Opportunity object is the Main Object to be used when creating the offer template, while Account is one of the related object and Opportunity Line Item is one of the Related List of the main object.
What is Additional Data Sources?
When you want to use data from difference sources, which is not easily accessible via the Main Objects, configuration Additional Data Sources is required. The added data will be visible in the Data Model for mapping to templates. The Additional Data Sources must relate to the Main Object via Parent Object, WHERE condition, or have a valid ID
How does the Data Model look like?
The data model in Dynamo depends on the main object and its relationship with other objects in Salesforce. When mapping data into templates, you can see list of data objects, all the fields, related objects and related lists in the Data Model as tree structure in the Data Source view
If you choose to open the list of Fields of the object, you can access to data of the related objects via the preferred fields. In the example, Account ID is preferred field to access related Account data of the Opportunity record
When click to the preferred field, you will open the data structure of the Account object and ensure to map the right Account data to the template