Quota Upgrades Cumulative or Absolute?


I'm wondering whether the quota upgrade functions in Upgrades are cumulative or absolute. For example, let's say that we offer 1, 2, 3, 4, and 5 GB quota upgrades using the Upgrades plugin and its built-in quota plugins.

If a user subscribes to the 2 GB upgrade (for example), does that add 2 GB to whatever the user has now, or does it simply take them from where they are to a 2 GB quota? I'm particularly interested in what would happen if a user did the 2 GB upgrade and then needed an additional 2 GB of space per month. Could they simply subscribe to the 2 GB upgrade again and add 2 GB to what they already have (for a 4 GB quota, give or take a little)?