From Magento 2-Jet extension for uploading the variant product .
If a merchant wants to upload a variant product say a shirt(S1) having size as attribute(large,medium,small).
So, he will create 3 variation products (S1-large), (S1-Medium), (S1-small) .
Now when he will upload this product on to jet the product which has been uploaded first will be the parent product and the remaining 2 products will be child product.
So, if (S1-Medium) product is uploaded first on jet then this will be the parent product and (S1-large), (S1-small) product will be the child product of (S1-Medium).
Note- The Product S1 will not be uploaded on jet and only (S1-large), (S1-Medium), (S1-small) these products will be uploaded on jet.