After your customer makes an order, you need to ship it. This process is referred to in Shopify as order fulfillment.

Partial Fulfillment is when you are trying to deliver a part of the customer's order. To enable partial fulfillment, you must also map the item SKU and quantity column. 

For example,

A customer ordered 3 pants with SKU XYZ0168 and 2 Sports Shoes with SKU XYZ0163.

But when you try to fulfill that order, you realize that the postage/shipping of both SKUs is different. 

So you want to update the tracking number to the customer separately.

By mapping the Item SKU during source setup, Fulfill Sync can perform partial fulfillment. You must map the item quantity using the column index in the example above using column index 1.