With the perpetual licensing model (before November 2, 2015), as a customer, you would buy a perpetual license to the product and then pay for upgrades to receive new versions.
This model applies to TeamCity, YouTrack, Hub, and Upsource.
With the subscription-based licensing model (after November 2, 2015):
If purchasing monthly:
You will have usage rights to all releases while your subscription is active. If you pay for 12 consecutive months, you will receive a perpetual fallback license for the exact version that was available when your 12 consecutive months term started. As your monthly subscription continues, your perpetual fallback license will grant you upgraded and renewed versions to 12 months prior.
If purchasing annually:
You will immediately get a perpetual fallback license for the exact version available at the time of purchase. This license will allow you to use that exact version of the software should you decide to discontinue the subscription after the year is up.
The perpetual fallback license is available whether you subscribe to a single product or the ‘All Products’ option.