FarApp looks up the carrier that's associated with the shipment method used on the Item Fulfillment and syncs that carrier in the Fulfillment Sync for the order.
With that in mind, it's important to ensure that the shipment method on fulfillments and the carrier associated with that shipment method in NetSuite is the carrier that was actually used to ship the order. If the carrier assigned to the shipment method in NetSuite differs from the carrier used for the actual shipment then the tracking information the customer sees after receiving their shipment confirmation will not be correct.
Special attention should be paid to this when using FarApp to sync your orders to a 3PL for fulfillment. Because you can map shipment methods coming from your 3PL to a different shipment method in NetSuite, you will want to make sure the method you are mapping to in NetSuite uses the same carrier as the one used to with the shipment method in the 3PL.
FarApp allows you to place the Carrier value in a custom field to have FarApp pull the Carrier it sends from that custom field. The procedure for doing that is at