A base price for a subscription plan defines how much the customer will pay per subscription period regardless of any other factors.
Some subscriptions are no more than a recurring monthly or yearly fee, like gym memberships. Some professionals, such as lawyers, charge a retainer that must be paid in addition to any further fees that may be incurred. Cell phone providers might charge a monthly fee that includes a certain level of bandwidth, and then charge more if that limit is exceeded.
If there are additional charges for overages, or if the price the customer pays changes depending on level of usage, then metered variables and metered variable tier types come into play.