Shiptheory offers the option of which Shopify reference you would like to use as the Order ID in Shiptheory. To select, go to your Shopify settings in Shiptheory, click on Advanced Settings in the bottom left and scroll down to Order ID Reference Type:
This is the normal order reference ID in Shopify, usually preceded by "#". Indicated in the image below as (1).
Note: If you create multiple fulfillments for one order, this reference will be the same for all fulfillments.
This is a fulfillment-specific reference, consisting of the order name (above) and appended with "F" and the fulfillment number. Indicated in the image below as (2).
Should you have any further questions, please get in contact with the Shiptheory support team who will be happy to help!