With subscribing for a fixed-term package, you will have access to the subscription content for the fixed number of days the package is defined with. For example if you subscribe for a 1-month package, you can use the content for 30 days starting from the day of your subscription.
You can extend all your fixed-term subscriptions for a maximum of 30 days and a fee of 7 USD per day, provided that you do so before your package expires. You can extend several times but the sum of the extended date cannot exceed 30 days. The amount of time you can extend each time is 1 day minimum.
Lower prices compared to custom subscription with the same number of days is the benefit of fixed-term packages.