Subtitles licenses are permanent and cover all the updates for the current major version. In other words, if you buy Subtitles 3, you will receive all updates for free until Subtitles 4 is released. At that point, you might choose to upgrade to Subtitles 4 at a reduced price, or continue to use Subtitles 3.
Note, however, that there are no current plans for a version 4. Upgrades with significant new features are still being released as part of version 3. Also, even if a version 4 is ever released, version 3 will still continue to be supported and receive bugfixes, but not new features.
Each license allows using Subtitles on up to 3 different computers at the same time. If you have any special requirements or want a license with a higher number of simultaneous activations, please contact us.
No, the licenses are not cross platform. In this case, you will need a license for Subtitles for Mac OS X and another one for Subtitles for Windows.