Quota Upgrades Cumulative or Absolute?

Hi,

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)?

Thanks,

Mark