Shuttle normally sends your Shopify stock to Etsy. In some cases Shopify has no usable number to send. A default stock quantity gives Shuttle a number to use in those cases.
When you transfer or update a product, Shuttle uses your default quantity if:
Shopify isn't tracking stock for the product.
The product is set to keep selling when out of stock in Shopify, and its quantity is 0 or less. Your Etsy listing stays available instead of selling out.
Shuttle is updating an Etsy listing that has one quantity for the whole listing, and the Shopify product has more than one variant.
In every other case, Shuttle sends your Shopify stock.
To use this option
Go to Product Settings → Shopify to Etsy → Stock Quantity.
Enter your default quantity, from 1 to 999. To always use your Shopify stock, leave the field empty.
Save your settings.
Tip: Some Etsy listings have one quantity for the whole listing. If you want those listings to get your lowest Shopify variant quantity, see Avoid Overselling on Etsy by Assigning Lowest Shopify Variant Quantity. If you use that option, leave the default quantity empty. Otherwise, products with more than one variant get the default quantity.