Open the Subscriptions category in the Send to picker:

• All Users With Active Subscription, All Users With Paused Subscription, All Users With Cancelled Subscription, All Users Whose Subscription Failed To Renew – by subscription status.
• One entry per subscription plan group – active subscribers of that plan group.
• Subscription customers who haven't select meals for upcoming deliveries – subscribers who still need to choose meals for the next delivery date.

Combine with Don't send to to exclude, for example, paused subscribers from an active-subscriber promotion. For ongoing reminders consider a Flow on the Subscription: Meals To Choose Reminder or Subscription: Renew Error trigger instead of repeated campaigns.