When building actions, you may occasionally see fields with names containing the words "source list". This field is basically a way for you to avoid having to create a repeat action to process a list, and allows you to map pills from a list directly into another list within an object. For example, in a QuickBooks invoice, you have your Line item fields. Usually, this means that if you have a list, you will need to create a repeat step, where for each item in the list, add a new row to an invoice. Now, all you need to do is map the list pill into the source list, and map the relevant fields underneath the source list. In the example below, I am mapping the opportunity product list from Salesforce to the sales line of a QuickBooks, so I simply take the List pill, in this case [Opportunity Product] and map it into the source list. After which, I simply map the relevant pills from Salesforce into their equivalent fields in the Invoice (eg. Line description > description, Amount > Total Price), and the action will automatically run through the entire list and add rows for each item in the source list.