What's the difference of sourcing mileage and non-sourcing mileage?

Good question, this doesn't have tax implications, just reporting implications for your business.

The "Sourcing Mileage" account will show up under the Cost of Sales section on your P&L, while "Non-Sourcing Mileage" will show up in the "Other Business Expenses" section of the P&L. You can learn more about the organization of the P&L by running the report and hovering over the info icon.

The principle is that if mileage is directly related to your sales process, you want to mark it as "Sourcing Mileage". Mileage that is not directly related to your sales process should be marked as "Non-Sourcing Mileage".