Every subscription has a subscription number.
The subscription number is used to unique identifier, so you can easily locate it.
As a Shopify merchant, you can view a customer's subscription number.
From Shopify admin:
From your Shopify admin, click Customers
2. Click the customer you want to view subscription information for
3. In the Active product subscriptions section, click View subscription. This will take you to the subscription information in Ongoing
4. Here you'll see a subscription number listed, starting with a hashtag #