Billing API

[xechungx]

I am trying to integrate with Shopify Billing API and implement the webhook for subscription update. (event: APP_SUBSCRIPTIONS_UPDATE). Do you have any spec about the payload sent with the event? From the following blog post: https://www.shopify.in/partners/blog/billing-api-graphql, it mentioned there are following status for a subscription:
Active, Declined, Expired, Canceled, or Frozen

I wonder do you have some documentation explain what will trigger the status transition? Thanks.