A customer is someone who currently pays you money for a product or service. This is different from a user. You can use a service like Facebook and not be a customer.
So for the purpose of subscriptions the following applies:
- Someone on a trial is not a customer.
- Someone who has a free subscription is not a customer. They are a lead that you hope will turn into a customer at some point in the future.
- Someone who pays a once off charge is not a customer as there is no recurring income.
- Someone who has a subscription starting in the future is not a customer yet.