When several variations stand for the same physical item, they should always show the same stock. The Update stock levels for all duplicate SKUs option takes care of this: a quantity change on one SKU is copied to every other variation with the same SKU, on both Etsy and Shopify.

The products don't have to be matched to each other. Update stock levels for all duplicate SKUs works whether or not they are matched.

The Stock Syncer settings have the same option under a different name. Go to Settings, then Stock Syncer, then General, and look in the Sync stock of duplicate SKUs card for Update all duplicate SKUs.

To use the Stock Syncer settings, you need the Growth plan or higher.

If you want duplicate SKUs synced only within the same product and its match, use Limit duplicate SKU syncing to the updated product instead of Update stock levels for all duplicate SKUs.

Only one of the two options can be on at a time. Turning on Update stock levels for all duplicate SKUs turns off Limit duplicate SKU syncing to the updated product.

See Syncing Duplicate SKUs Within the Same Product.

Say you have an Etsy listing with three variations, with the SKUs AAA, BBB and CCC.

On Shopify, those aren't three variants of one product. They are three separate products, one for each SKU: AAA, BBB and CCC.

You also sell a kit on Etsy, and one of the kit's variations has the SKU BBB too.

With Update stock levels for all duplicate SKUs turned on, a sale of the BBB variation on Etsy means Shuttle also updates:

When Update stock levels for all duplicate SKUs is on, Shuttle updates up to 10 products on each platform for each SKU.