_subscription_frequency. When defined at the cart level, the subscription will apply to all subscription items in the cart.
_include_all_items_in_subscriptioncart-level attribute serves this purpose. When present, there is no need to include any
_subscription_line_itemline-item proiperties - instead, all of the carts items will be automatically included in the subscription.
_submarine_initial_scheduled_atcart-level attribute. This should be set to ISO 8601-formatted time that the first subscription order should be generated on, e.g.
truein the resulting subscription.