Implementing in the Shopify platform:
1. Editor page URL resembles: https://checkout.shopify.com/1234567890/checkouts/1234567890abcdefghijklmn
...where the last serial number is a cart id, which is fine for selecting elements and creating variants. Leaving the cart-id off gives you a 404 so you *have* to include it to use the Variant-builder plugin. BUT... (see #2)
2. "Targeting" automatically adds a "When" rule: "URL matches" with the full URL above, which is only good for exactly one person's cart (via aforementioned cart-id). If there was an "OR" operator you could still logically work around this but the only logic operator is "AND".
So I'm stuck either with being able to create Variations because with an Editor page that is too specific to create appropriate targeting rules, or an 404 for the Editor page which doesn't let me create variations.