Clarification was added to V3 Catalog API on creating products and product variants. Included are examples on how to create a product with variants, adding variant options and use cases for creating variant options.

Create a Product with Variants

{ "name": "BigCommerce Coffee Mug", "price": "10.00", "categories":[ 23,21 ], "weight": 4, "type": "physical", "variants":[ { "sku": "SKU-BLU", "option_values":[ { "option_display_name": "Mug Color", "label": "Blue" } ] }, { "sku": "SKU-GRAY", "option_values":[ { "option_display_name": "Mug Color", "label": "Gray" } ] } ] }