title: Why do some variants miss the subscription-related fields like requires_selling_plan?
body: <p>According to <a href="https://shopify.dev/api/liquid/objects/variant#variant-requires_selling_plan" rel="nofollow noreferrer">Shopify variant liquid object</a>, there are subscription-related fields like requires_selling_plan, selling_plan_allocations, etc.</p>
  <p>But I can't find these fields on my variants. I tried to add subscription plans to my products with Recharge or Awtomatic subscription apps. But these fields are still not found. How can I add these fields to my variants? And why are they missing? I can't find any information from the subscription docs.</p>