TL;DR Please add/update documentation explaining these Shopify Fulfillment features.
Orders API:
In the past people have asked about Shopify’s carrier “code” and “title”, as they’re present on an Orders shipping_lines,
(Orders JSON => “show 67 hidden fields” => shipping_lines => “Show shipping_lines properties”,) and are the primary way the chosen shipping company is sent to the fulfillment service. This “code” and “title” shown are not included in the CarrierService resource, the supposed source of the data, and the CarrierService “id” and “name” are not what is copied to an Order’s shipping_lines. All quite confusing.
Fulfillments API:
A list of Shopify-natively-recognized carrier codes does exist in the docs! (2023-01) And it lives in an extremely odd place.
(Fulfillment JSON => “Show 5 hidden fields” => tracking_company => “Show tracking_company properties”)
It’s unclear whether this list is even valid, or if it’s somehow derived from Shopify’s supported CarrierServices, or how Shopify would handle carrier codes not in the list, etc.
It does something! There is slick-but-totally-undocumented tracking number pattern matching and tracking URL generation. (To match tracking to a guessed carrier and help ensure valid tracking URLs are present. A requirement likely born from “notify” email issues.)
All quite confusing.