When you are mapping your fields in Formstack Sync, you'll often see something like Contact > Account or Contact > Opportunity or even Opportunity > Account if you are able to map the Opportunity type object directly.
What this means is you can read data from related objects to the root object in the mapping. So in the case of something like Contact > Account, the root object here is the Contact and the related object is the Account. The fields that show up in the Field drop down will display all of the available fields found on the related Account object. You can then map this back over to the Contact object in the other system.
One thing to keep in mind is that these fields on the related objects are READ ONLY fields. In other words, you can only read the data but you can't write to these fields from the other system.
There is only one type of scenario where you can write to a related object field and that's when the field is required to be populated when creating a brand new Contact in the given system. A common example is when your CRM requires a Company be created when you are creating a new Contact. Therefore you will have to map the Company Name field in that system through the Contact. The screenshot below is an example of what this setup will look like in your mapping.
Here the Company Name field out of HubSpot will be allowed to write to the Company Name field in the Netsuite Lead Company object.
This type of mapping where you are grabbing data from a related object to then live on the root object in the other system is valuable for when you are unable to map different objects directly with each other (Learn more about Mapping Relationships here). This way you can still run processes off of this data in the system where you just have the one object, such as a Contact.