To assign a SKU to a specific product variant on 3DCart, follow these steps.

First, ensure that each option set (e.g. Size or Color) of a product is a "Required" option.

1, Navigate to Products > Product List
2, Find the product you'd like to edit and click on its name
3, Click on the "Options" tab
4, Next, on the "Edit option" window, click edit at the top-right an option set table

5, Check the "Required" option

6, Save
7, Repeat this for each option set.

Next, enable Advanced Options for the product.

1, At the top of the page, click on Actions > Advanced Options

2, Next, select the checkboxes to the left of the variant rows
3, Enter unique SKU in the field under the "Code" column for each variant 

4, Save
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You can learn more about how to set up product options here.