To define what fields from the item selected will go where in the form. In the field settings of your field in the form template, use the "Add Item" button to begin.

The left dropdown in the information available for each item from the database. The right dropdown is the form field the information should go to. You can add as many items as you need by clicking "Add Item" and matching the item data with the form fields.

In the example, the Name, Description and Sales Price are being brought over from the database and inserted into the ProductName, ProductDescription and unitCost fields respectively.

Once you have set up the look up, Click the save Draft, and then Deploy / Publish Template buttons to have the changes available when starting a new form. 

For this example you can see that when you click into the product search field, a searchable dropdown will appear with the products that match your filter, and with the information selected during configuration:

And when an item is selected from that list, the fields will populate as configured:

Using this method, you can bring information over from your Xero account, or from custom databases created in Lookup Databases on Formitize.