These are one of the most important settings in Syncee. In the last section of your Mapping step in the wizard, you can select the data that Syncee will use to identify the products in your store that you have uploaded previously. You only have to set this, if you didn't upload the products with Syncee, or if you uploaded them in another task.
If you select "Do not compare" Syncee will upload and update everything from your source file without searching for duplicate products in your store. If you know that you already have products in your Shopify that is also included in the source file, please choose the field Syncee should use to identify them. It can be by product SKU, Barcode, Handle (The name of the product in your URL) or by product title.
Then, you can select how you would like Syncee to identify product variants from your store. You can set it to "Do not compare" and Syncee will upload or update everything from your source file without comparing the products to your Shopify store inventory. Or, you can choose to identify them by variant SKU or variant Barcode.
If you would like to know more about the Update options, learn more here.