Subscribers may renew automatically, their access may expire after some time or usage threshold, or, in some cases, they may be on a "pay as you go" model.
Though mostly associated with paid access, many social media, SaaS, or gaming vendors use a freemium business model that can grow a large number of non-paid subscribers. It's important to be able to distinguish between paid and non-paid, as behaviours and value can be very different.
Subscription businesses will often report on New Subscribers, Active Subscribers, and Cancelled or Churned Subscribers.