The tags show which is reservation based and which is always on. Find the sandbox you wish to reserve and click the launch tab. On the next page you can select the duration or except the defaults. Click launch on this page and the sandbox will go through the set up process send two emails (one for setting up and once set up your vpn details).
If the launch button is greyed out, this means all reservations are currently in use for the sandbox.
Hope this helps.
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