Zoho intelligently designed all the apps in the Finance Suite to share the same database.
Thus, as you begin using Subscriptions, you may immediately work with all your existing Customers from Zoho Invoice, Inventory, etc. because the data is already there!
Likewise, when your clients make payments on their subscriptions, those invoices will show up in Zoho Books. Take a look!
If you are just using Subscriptions itself, some of what we discuss is not relevant (syncing of data between the apps, Accounts, etc.) but most of the general principles apply.
