A subscription pack describes the payment schedule (monthly or annually), payment dates, and payment methods of (some or all of) your Toolbox licenses.
The subscription pack also serves as a way to co-term subscriptions added at different times, to provide a shared expiration date. When a subscription pack comes due annually, payment is charged automatically if you have a card or PayPal account linked to the subscription pack, or a reminder email is sent if there is no payment data linked to the subscription pack.
Ex. A new subscription pack begins on January 24, 2016, with the purchase of 10 subscriptions. On July 24, 2016, 10 new subscriptions are purchased and added to the same subscription pack. As the subscription pack comes due on January 23, 2017, all subscriptions expiring during the next year for this subscription pack would be included with the renewal. The renewal would include full-year subscriptions for 10 licenses (January 24, 2017 - January 23, 2018) and 6-month subscriptions for 10 licenses (July 24, 2017 - January 23, 2018). At this point, all subscriptions would expire and co-term on the same date.
Having more than one subscription pack in your account is possible. Companies may have several subscription packs to arrange billing schedules, payment totals and payment methods for groups of licenses according to their needs.