A subscription is a business model where a company can sell their services on a termed (recurring basis). It reflects the agreement between the company and the consumer where termed fees (monthly or quarterly etc) are applied for services such as TV packages, broadband services, fleet usage, etc.
The following are the key business components of a subscription:

Termed services(billed on a recurrence basis) such as SVODs or TV channels,
Usage services (billed based on usage) such as TVODs (i.e.movies) (Optional)
Hardware on which service signals are sent to; referred to as Installed Items in CRM.COM (Optional)
Fees such as start up fee (Optional)

In CRM.COM subscriptions are categorized to Normal and Prepaid which essentially describes the way they are billed. Normal subscriptions are funded by an account controlled by a credit limit and services are rated and billed either before or after their consumption In prepaid subscription customers subscribe for a set of services and pay them in advance, prior their consumption using wallet funds.

If your business is associated with usage based subscriptions, i.e, users billed based on the usage (films, internet, gas etc.) you can either work with normal or prepaid based models.