You can see this setting on your Amazon integrations page, in the Advanced Settings tab.
Here is each option and what it means.
Rarely, do you want to choose this option. This means LM will not touch the QTY for the non primary FBM SKU. This can be an issue, as the SKUs won't share QTY. So if one sells, it will not affect the QTY of the other SKU, thus leading to overselling. However, if your intent is to have the QTYs of the primary FBM SKU and the non primary FBM SKU be separate from each other, this is what you would want to choose.
2. Sync Zero to Duplicates
This is the typical option most people use. It means we will only sync QTY to one FBM SKU, not both, preventing from overselling. The QTY is synced to the primary FBM SKU, which can be changed on your Manage Duplicate Listings page.
3. Copy to Duplicates
This is used if you want to maintain the same QTY across your duplicate FBM SKUs. LM will take the QTY from the primary FBM SKU and sync it to the other non-primary fbm skus.