Yeah, this is a bit tricky, but I’ll try to explain.
When Skuid loads, it gets everything it needs for all of your models. This is any metadata about the objects (picklist values, display types, etc), as well as the actual records contained in the model. In the case of your NewSalesTerritory model, it starts out with no data. Once the page is loaded, Skuid does not communicate with the server again until a model requests a reload or a save.
In your case, when the popup is opened and a Firm record is searched for, Skuid makes a request to salesforce to bring back data on all of the Firms that match your search criteria. The “searchfields” configuration tells Skuid which fields to query on the Firms that you’re searching for. Once you select a Firm, Skuid automatically transfers data from the search response into your NewSalesTerritory model. If you don’t tell Skuid to search for the “BillingPostalCode” field, it will not be able to transfer it over to your NewSalesTerritory model.
Anyways, I hope that helps.