If you set Eligible subscriptions as New and existing subscriptions, this means the new customers can apply the coupon during the checkout process, and the existing customers can apply the coupon within MyAccount. In short, everybody can apply the coupon on their subscription.
How to set up coupon campaigns for all subscriptions on the dashboard.
Go to the Offers & Coupons section of the platform, and then to the Create Coupons section of Coupon Management.
Here you can create your campaign as usual, with one small change. When setting the offers for Campaign applicable on, choose subscription offers (recurring billing) from the drop-down, and the New & existing subscriptions.
Now you can create a campaign that will allow your new and existing customers to apply a coupon.