Subscription means you will pay for your campaign on a weekly schedule.
An example: When the campaign starts your card will be authorized the weekly budget. After seven days, the authorized amount is adjusted, and you pay for what was actually delivered. This cycle continues weekly until the end date or until you cancel your subscription.
For Subscription you set Start date and can choose between fixed or open End date.
Please note: Depending on the platform setup the Subscription budget model might not be available.