Advance source allow you to add custom description and display your product / variant custom metafield values. The advance source can only read metafields from Products and Variants.
To add a metafield, add :: in front of your metafield namespace and key. For example:
You can also add custom text together with your metafields:
The product dimension is ::product.dimension
The advance field also supports multiple metafields:
The product dimension is ::product.dimension and weight is ::product.weight_in_kg
If the product has variant, the value of the metafields will change accordingly when the variant of the product is selected.