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. 

For example,

A customer ordered 5 pants with SKU 1001 and 3 jackets with SKU 2002.

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

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

By mapping the Item SKU during source setup, Fulfill Sync is able to perform partial fufillment.