Merge Schema Items is as option that allows you to merge all selected schema items in a single post type. i.e
One post has Book and Product schema selected. if you switch on the " Merge selected schema items" option. this page schema will show up in Google Structured data as Book/Product.
Merge will allow you to merge two different items such as Course/Product, Movie/Product, Game/Product.
Merge with WooCommerce Product Schema
This option is very useful with WooCommerce products when you want to add second item to a product but you still want it to stay product and you don't want two different schema types shown in the same page.
However there is one more step that you have to take. You need to create a schema item with product schema type. You should add at least on property in this item, and publish.
When you are selecting schema items in the product edit page make sure to select that schema item as well.
now you can switch on the merge option and it will extend the WooCommerce Schema.